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cgroups: update comments in cpuset.c
Some of the comments in kernel/cpuset.c were stale following the transition to control groups; this patch updates them to more closely match reality. Signed-off-by: Paul Menage <menage@google.com> Acked-by: Paul Jackson <pj@sgi.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Paul Menage
Linus Torvalds
parent
58f4790b73
commit
2df167a300
1 changed files with 43 additions and 85 deletions
128
kernel/cpuset.c
128
kernel/cpuset.c
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@ -65,7 +65,7 @@
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*/
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int number_of_cpusets __read_mostly;
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/* Retrieve the cpuset from a cgroup */
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/* Forward declare cgroup structures */
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struct cgroup_subsys cpuset_subsys;
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struct cpuset;
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@ -167,17 +167,17 @@ static inline int is_spread_slab(const struct cpuset *cs)
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* number, and avoid having to lock and reload mems_allowed unless
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* the cpuset they're using changes generation.
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*
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* A single, global generation is needed because attach_task() could
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* A single, global generation is needed because cpuset_attach_task() could
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* reattach a task to a different cpuset, which must not have its
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* generation numbers aliased with those of that tasks previous cpuset.
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*
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* Generations are needed for mems_allowed because one task cannot
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* modify anothers memory placement. So we must enable every task,
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* modify another's memory placement. So we must enable every task,
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* on every visit to __alloc_pages(), to efficiently check whether
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* its current->cpuset->mems_allowed has changed, requiring an update
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* of its current->mems_allowed.
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*
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* Since cpuset_mems_generation is guarded by manage_mutex,
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* Since writes to cpuset_mems_generation are guarded by the cgroup lock
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* there is no need to mark it atomic.
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*/
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static int cpuset_mems_generation;
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@ -189,17 +189,20 @@ static struct cpuset top_cpuset = {
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};
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/*
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* We have two global cpuset mutexes below. They can nest.
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* It is ok to first take manage_mutex, then nest callback_mutex. We also
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* require taking task_lock() when dereferencing a tasks cpuset pointer.
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* See "The task_lock() exception", at the end of this comment.
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* There are two global mutexes guarding cpuset structures. The first
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* is the main control groups cgroup_mutex, accessed via
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* cgroup_lock()/cgroup_unlock(). The second is the cpuset-specific
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* callback_mutex, below. They can nest. It is ok to first take
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* cgroup_mutex, then nest callback_mutex. We also require taking
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* task_lock() when dereferencing a task's cpuset pointer. See "The
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* task_lock() exception", at the end of this comment.
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*
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* A task must hold both mutexes to modify cpusets. If a task
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* holds manage_mutex, then it blocks others wanting that mutex,
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* holds cgroup_mutex, then it blocks others wanting that mutex,
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* ensuring that it is the only task able to also acquire callback_mutex
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* and be able to modify cpusets. It can perform various checks on
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* the cpuset structure first, knowing nothing will change. It can
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* also allocate memory while just holding manage_mutex. While it is
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* also allocate memory while just holding cgroup_mutex. While it is
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* performing these checks, various callback routines can briefly
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* acquire callback_mutex to query cpusets. Once it is ready to make
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* the changes, it takes callback_mutex, blocking everyone else.
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@ -215,60 +218,16 @@ static struct cpuset top_cpuset = {
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* The task_struct fields mems_allowed and mems_generation may only
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* be accessed in the context of that task, so require no locks.
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*
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* Any task can increment and decrement the count field without lock.
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* So in general, code holding manage_mutex or callback_mutex can't rely
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* on the count field not changing. However, if the count goes to
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* zero, then only attach_task(), which holds both mutexes, can
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* increment it again. Because a count of zero means that no tasks
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* are currently attached, therefore there is no way a task attached
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* to that cpuset can fork (the other way to increment the count).
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* So code holding manage_mutex or callback_mutex can safely assume that
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* if the count is zero, it will stay zero. Similarly, if a task
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* holds manage_mutex or callback_mutex on a cpuset with zero count, it
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* knows that the cpuset won't be removed, as cpuset_rmdir() needs
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* both of those mutexes.
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*
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* The cpuset_common_file_write handler for operations that modify
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* the cpuset hierarchy holds manage_mutex across the entire operation,
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* the cpuset hierarchy holds cgroup_mutex across the entire operation,
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* single threading all such cpuset modifications across the system.
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*
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* The cpuset_common_file_read() handlers only hold callback_mutex across
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* small pieces of code, such as when reading out possibly multi-word
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* cpumasks and nodemasks.
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*
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* The fork and exit callbacks cpuset_fork() and cpuset_exit(), don't
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* (usually) take either mutex. These are the two most performance
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* critical pieces of code here. The exception occurs on cpuset_exit(),
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* when a task in a notify_on_release cpuset exits. Then manage_mutex
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* is taken, and if the cpuset count is zero, a usermode call made
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* to /sbin/cpuset_release_agent with the name of the cpuset (path
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* relative to the root of cpuset file system) as the argument.
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*
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* A cpuset can only be deleted if both its 'count' of using tasks
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* is zero, and its list of 'children' cpusets is empty. Since all
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* tasks in the system use _some_ cpuset, and since there is always at
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* least one task in the system (init), therefore, top_cpuset
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* always has either children cpusets and/or using tasks. So we don't
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* need a special hack to ensure that top_cpuset cannot be deleted.
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*
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* The above "Tale of Two Semaphores" would be complete, but for:
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*
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* The task_lock() exception
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*
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* The need for this exception arises from the action of attach_task(),
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* which overwrites one tasks cpuset pointer with another. It does
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* so using both mutexes, however there are several performance
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* critical places that need to reference task->cpuset without the
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* expense of grabbing a system global mutex. Therefore except as
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* noted below, when dereferencing or, as in attach_task(), modifying
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* a tasks cpuset pointer we use task_lock(), which acts on a spinlock
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* (task->alloc_lock) already in the task_struct routinely used for
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* such matters.
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*
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* P.S. One more locking exception. RCU is used to guard the
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* update of a tasks cpuset pointer by attach_task() and the
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* access of task->cpuset->mems_generation via that pointer in
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* the routine cpuset_update_task_memory_state().
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* Accessing a task's cpuset should be done in accordance with the
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* guidelines for accessing subsystem state in kernel/cgroup.c
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*/
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static DEFINE_MUTEX(callback_mutex);
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@ -361,15 +320,14 @@ static void guarantee_online_mems(const struct cpuset *cs, nodemask_t *pmask)
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* Do not call this routine if in_interrupt().
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*
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* Call without callback_mutex or task_lock() held. May be
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* called with or without manage_mutex held. Thanks in part to
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* 'the_top_cpuset_hack', the tasks cpuset pointer will never
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* called with or without cgroup_mutex held. Thanks in part to
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* 'the_top_cpuset_hack', the task's cpuset pointer will never
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* be NULL. This routine also might acquire callback_mutex and
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* current->mm->mmap_sem during call.
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*
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* Reading current->cpuset->mems_generation doesn't need task_lock
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* to guard the current->cpuset derefence, because it is guarded
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* from concurrent freeing of current->cpuset by attach_task(),
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* using RCU.
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* from concurrent freeing of current->cpuset using RCU.
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*
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* The rcu_dereference() is technically probably not needed,
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* as I don't actually mind if I see a new cpuset pointer but
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@ -431,7 +389,7 @@ void cpuset_update_task_memory_state(void)
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*
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* One cpuset is a subset of another if all its allowed CPUs and
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* Memory Nodes are a subset of the other, and its exclusive flags
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* are only set if the other's are set. Call holding manage_mutex.
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* are only set if the other's are set. Call holding cgroup_mutex.
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*/
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static int is_cpuset_subset(const struct cpuset *p, const struct cpuset *q)
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@ -449,7 +407,7 @@ static int is_cpuset_subset(const struct cpuset *p, const struct cpuset *q)
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* If we replaced the flag and mask values of the current cpuset
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* (cur) with those values in the trial cpuset (trial), would
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* our various subset and exclusive rules still be valid? Presumes
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* manage_mutex held.
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* cgroup_mutex held.
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*
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* 'cur' is the address of an actual, in-use cpuset. Operations
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* such as list traversal that depend on the actual address of the
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@ -483,7 +441,10 @@ static int validate_change(const struct cpuset *cur, const struct cpuset *trial)
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if (!is_cpuset_subset(trial, par))
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return -EACCES;
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/* If either I or some sibling (!= me) is exclusive, we can't overlap */
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/*
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* If either I or some sibling (!= me) is exclusive, we can't
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* overlap
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*/
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list_for_each_entry(cont, &par->css.cgroup->children, sibling) {
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c = cgroup_cs(cont);
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if ((is_cpu_exclusive(trial) || is_cpu_exclusive(c)) &&
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@ -744,7 +705,7 @@ static inline int started_after(void *p1, void *p2)
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* @tsk: task to test
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* @scan: struct cgroup_scanner contained in its struct cpuset_hotplug_scanner
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*
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* Call with manage_mutex held. May take callback_mutex during call.
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* Call with cgroup_mutex held. May take callback_mutex during call.
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* Called for each task in a cgroup by cgroup_scan_tasks().
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* Return nonzero if this tasks's cpus_allowed mask should be changed (in other
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* words, if its mask is not equal to its cpuset's mask).
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@ -847,11 +808,11 @@ static int update_cpumask(struct cpuset *cs, char *buf)
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* Temporarilly set tasks mems_allowed to target nodes of migration,
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* so that the migration code can allocate pages on these nodes.
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*
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* Call holding manage_mutex, so our current->cpuset won't change
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* during this call, as manage_mutex holds off any attach_task()
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* Call holding cgroup_mutex, so current's cpuset won't change
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* during this call, as cgroup_mutex holds off any attach_task()
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* calls. Therefore we don't need to take task_lock around the
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* call to guarantee_online_mems(), as we know no one is changing
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* our tasks cpuset.
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* our task's cpuset.
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*
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* Hold callback_mutex around the two modifications of our tasks
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* mems_allowed to synchronize with cpuset_mems_allowed().
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@ -896,7 +857,7 @@ static void cpuset_migrate_mm(struct mm_struct *mm, const nodemask_t *from,
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* the cpuset is marked 'memory_migrate', migrate the tasks
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* pages to the new memory.
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*
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* Call with manage_mutex held. May take callback_mutex during call.
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* Call with cgroup_mutex held. May take callback_mutex during call.
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* Will take tasklist_lock, scan tasklist for tasks in cpuset cs,
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* lock each such tasks mm->mmap_sem, scan its vma's and rebind
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* their mempolicies to the cpusets new mems_allowed.
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@ -1009,7 +970,7 @@ static int update_nodemask(struct cpuset *cs, char *buf)
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* tasklist_lock. Forks can happen again now - the mpol_copy()
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* cpuset_being_rebound check will catch such forks, and rebind
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* their vma mempolicies too. Because we still hold the global
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* cpuset manage_mutex, we know that no other rebind effort will
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* cgroup_mutex, we know that no other rebind effort will
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* be contending for the global variable cpuset_being_rebound.
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* It's ok if we rebind the same mm twice; mpol_rebind_mm()
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* is idempotent. Also migrate pages in each mm to new nodes.
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@ -1024,7 +985,7 @@ static int update_nodemask(struct cpuset *cs, char *buf)
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mmput(mm);
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}
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/* We're done rebinding vma's to this cpusets new mems_allowed. */
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/* We're done rebinding vmas to this cpuset's new mems_allowed. */
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kfree(mmarray);
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cpuset_being_rebound = NULL;
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retval = 0;
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@ -1038,7 +999,7 @@ int current_cpuset_is_being_rebound(void)
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}
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/*
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* Call with manage_mutex held.
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* Call with cgroup_mutex held.
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*/
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static int update_memory_pressure_enabled(struct cpuset *cs, char *buf)
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@ -1059,7 +1020,7 @@ static int update_memory_pressure_enabled(struct cpuset *cs, char *buf)
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* cs: the cpuset to update
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* buf: the buffer where we read the 0 or 1
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*
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* Call with manage_mutex held.
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* Call with cgroup_mutex held.
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*/
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static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs, char *buf)
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@ -1193,6 +1154,7 @@ static int fmeter_getrate(struct fmeter *fmp)
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return val;
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}
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/* Called by cgroups to determine if a cpuset is usable; cgroup_mutex held */
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static int cpuset_can_attach(struct cgroup_subsys *ss,
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struct cgroup *cont, struct task_struct *tsk)
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{
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@ -1540,7 +1502,8 @@ static int cpuset_populate(struct cgroup_subsys *ss, struct cgroup *cont)
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* If this becomes a problem for some users who wish to
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* allow that scenario, then cpuset_post_clone() could be
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* changed to grant parent->cpus_allowed-sibling_cpus_exclusive
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* (and likewise for mems) to the new cgroup.
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* (and likewise for mems) to the new cgroup. Called with cgroup_mutex
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* held.
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*/
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static void cpuset_post_clone(struct cgroup_subsys *ss,
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struct cgroup *cgroup)
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@ -1564,11 +1527,8 @@ static void cpuset_post_clone(struct cgroup_subsys *ss,
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/*
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* cpuset_create - create a cpuset
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* parent: cpuset that will be parent of the new cpuset.
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* name: name of the new cpuset. Will be strcpy'ed.
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* mode: mode to set on new inode
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*
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* Must be called with the mutex on the parent inode held
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* ss: cpuset cgroup subsystem
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* cont: control group that the new cpuset will be part of
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*/
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static struct cgroup_subsys_state *cpuset_create(
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@ -1769,7 +1729,7 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
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* member tasks or cpuset descendants and cpus and memory, before it can
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* be a candidate for release.
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*
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* Called with manage_mutex held. We take callback_mutex to modify
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* Called with cgroup_mutex held. We take callback_mutex to modify
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* cpus_allowed and mems_allowed.
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*
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* This walk processes the tree from top to bottom, completing one layer
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@ -1910,7 +1870,7 @@ cpumask_t cpuset_cpus_allowed(struct task_struct *tsk)
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/**
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* cpuset_cpus_allowed_locked - return cpus_allowed mask from a tasks cpuset.
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* Must be called with callback_mutex held.
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* Must be called with callback_mutex held.
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**/
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cpumask_t cpuset_cpus_allowed_locked(struct task_struct *tsk)
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{
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@ -2247,10 +2207,8 @@ void __cpuset_memory_pressure_bump(void)
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* - Used for /proc/<pid>/cpuset.
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* - No need to task_lock(tsk) on this tsk->cpuset reference, as it
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* doesn't really matter if tsk->cpuset changes after we read it,
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* and we take manage_mutex, keeping attach_task() from changing it
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* anyway. No need to check that tsk->cpuset != NULL, thanks to
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* the_top_cpuset_hack in cpuset_exit(), which sets an exiting tasks
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* cpuset to top_cpuset.
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* and we take cgroup_mutex, keeping attach_task() from changing it
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* anyway.
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*/
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static int proc_cpuset_show(struct seq_file *m, void *unused_v)
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{
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