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cgroups: update comments in cpuset.c

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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 2008-02-07 00:14:45 -08:00 committed by Linus Torvalds
parent 58f4790b73
commit 2df167a300
1 changed files with 43 additions and 85 deletions
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128
kernel/cpuset.c
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@ -65,7 +65,7 @@
*/
int number_of_cpusets __read_mostly;
/* Retrieve the cpuset from a cgroup */
/* Forward declare cgroup structures */
struct cgroup_subsys cpuset_subsys;
struct cpuset;
@ -167,17 +167,17 @@ static inline int is_spread_slab(const struct cpuset *cs)
* number, and avoid having to lock and reload mems_allowed unless
* the cpuset they're using changes generation.
*
* A single, global generation is needed because attach_task() could
* A single, global generation is needed because cpuset_attach_task() could
* reattach a task to a different cpuset, which must not have its
* generation numbers aliased with those of that tasks previous cpuset.
*
* Generations are needed for mems_allowed because one task cannot
* modify anothers memory placement. So we must enable every task,
* modify another's memory placement. So we must enable every task,
* on every visit to __alloc_pages(), to efficiently check whether
* its current->cpuset->mems_allowed has changed, requiring an update
* of its current->mems_allowed.
*
* Since cpuset_mems_generation is guarded by manage_mutex,
* Since writes to cpuset_mems_generation are guarded by the cgroup lock
* there is no need to mark it atomic.
*/
static int cpuset_mems_generation;
@ -189,17 +189,20 @@ static struct cpuset top_cpuset = {
};
/*
* We have two global cpuset mutexes below. They can nest.
* It is ok to first take manage_mutex, then nest callback_mutex. We also
* require taking task_lock() when dereferencing a tasks cpuset pointer.
* See "The task_lock() exception", at the end of this comment.
* There are two global mutexes guarding cpuset structures. The first
* is the main control groups cgroup_mutex, accessed via
* cgroup_lock()/cgroup_unlock(). The second is the cpuset-specific
* callback_mutex, below. They can nest. It is ok to first take
* cgroup_mutex, then nest callback_mutex. We also require taking
* task_lock() when dereferencing a task's cpuset pointer. See "The
* task_lock() exception", at the end of this comment.
*
* A task must hold both mutexes to modify cpusets. If a task
* holds manage_mutex, then it blocks others wanting that mutex,
* holds cgroup_mutex, then it blocks others wanting that mutex,
* ensuring that it is the only task able to also acquire callback_mutex
* and be able to modify cpusets. It can perform various checks on
* the cpuset structure first, knowing nothing will change. It can
* also allocate memory while just holding manage_mutex. While it is
* also allocate memory while just holding cgroup_mutex. While it is
* performing these checks, various callback routines can briefly
* acquire callback_mutex to query cpusets. Once it is ready to make
* the changes, it takes callback_mutex, blocking everyone else.
@ -215,60 +218,16 @@ static struct cpuset top_cpuset = {
* The task_struct fields mems_allowed and mems_generation may only
* be accessed in the context of that task, so require no locks.
*
* Any task can increment and decrement the count field without lock.
* So in general, code holding manage_mutex or callback_mutex can't rely
* on the count field not changing. However, if the count goes to
* zero, then only attach_task(), which holds both mutexes, can
* increment it again. Because a count of zero means that no tasks
* are currently attached, therefore there is no way a task attached
* to that cpuset can fork (the other way to increment the count).
* So code holding manage_mutex or callback_mutex can safely assume that
* if the count is zero, it will stay zero. Similarly, if a task
* holds manage_mutex or callback_mutex on a cpuset with zero count, it
* knows that the cpuset won't be removed, as cpuset_rmdir() needs
* both of those mutexes.
*
* The cpuset_common_file_write handler for operations that modify
* the cpuset hierarchy holds manage_mutex across the entire operation,
* the cpuset hierarchy holds cgroup_mutex across the entire operation,
* single threading all such cpuset modifications across the system.
*
* The cpuset_common_file_read() handlers only hold callback_mutex across
* small pieces of code, such as when reading out possibly multi-word
* cpumasks and nodemasks.
*
* The fork and exit callbacks cpuset_fork() and cpuset_exit(), don't
* (usually) take either mutex. These are the two most performance
* critical pieces of code here. The exception occurs on cpuset_exit(),
* when a task in a notify_on_release cpuset exits. Then manage_mutex
* is taken, and if the cpuset count is zero, a usermode call made
* to /sbin/cpuset_release_agent with the name of the cpuset (path
* relative to the root of cpuset file system) as the argument.
*
* A cpuset can only be deleted if both its 'count' of using tasks
* is zero, and its list of 'children' cpusets is empty. Since all
* tasks in the system use _some_ cpuset, and since there is always at
* least one task in the system (init), therefore, top_cpuset
* always has either children cpusets and/or using tasks. So we don't
* need a special hack to ensure that top_cpuset cannot be deleted.
*
* The above "Tale of Two Semaphores" would be complete, but for:
*
* The task_lock() exception
*
* The need for this exception arises from the action of attach_task(),
* which overwrites one tasks cpuset pointer with another. It does
* so using both mutexes, however there are several performance
* critical places that need to reference task->cpuset without the
* expense of grabbing a system global mutex. Therefore except as
* noted below, when dereferencing or, as in attach_task(), modifying
* a tasks cpuset pointer we use task_lock(), which acts on a spinlock
* (task->alloc_lock) already in the task_struct routinely used for
* such matters.
*
* P.S. One more locking exception. RCU is used to guard the
* update of a tasks cpuset pointer by attach_task() and the
* access of task->cpuset->mems_generation via that pointer in
* the routine cpuset_update_task_memory_state().
* Accessing a task's cpuset should be done in accordance with the
* guidelines for accessing subsystem state in kernel/cgroup.c
*/
static DEFINE_MUTEX(callback_mutex);
@ -361,15 +320,14 @@ static void guarantee_online_mems(const struct cpuset *cs, nodemask_t *pmask)
* Do not call this routine if in_interrupt().
*
* Call without callback_mutex or task_lock() held. May be
* called with or without manage_mutex held. Thanks in part to
* 'the_top_cpuset_hack', the tasks cpuset pointer will never
* called with or without cgroup_mutex held. Thanks in part to
* 'the_top_cpuset_hack', the task's cpuset pointer will never
* be NULL. This routine also might acquire callback_mutex and
* current->mm->mmap_sem during call.
*
* Reading current->cpuset->mems_generation doesn't need task_lock
* to guard the current->cpuset derefence, because it is guarded
* from concurrent freeing of current->cpuset by attach_task(),
* using RCU.
* from concurrent freeing of current->cpuset using RCU.
*
* The rcu_dereference() is technically probably not needed,
* as I don't actually mind if I see a new cpuset pointer but
@ -431,7 +389,7 @@ void cpuset_update_task_memory_state(void)
*
* One cpuset is a subset of another if all its allowed CPUs and
* Memory Nodes are a subset of the other, and its exclusive flags
* are only set if the other's are set. Call holding manage_mutex.
* are only set if the other's are set. Call holding cgroup_mutex.
*/
static int is_cpuset_subset(const struct cpuset *p, const struct cpuset *q)
@ -449,7 +407,7 @@ static int is_cpuset_subset(const struct cpuset *p, const struct cpuset *q)
* If we replaced the flag and mask values of the current cpuset
* (cur) with those values in the trial cpuset (trial), would
* our various subset and exclusive rules still be valid? Presumes
* manage_mutex held.
* cgroup_mutex held.
*
* 'cur' is the address of an actual, in-use cpuset. Operations
* such as list traversal that depend on the actual address of the
@ -483,7 +441,10 @@ static int validate_change(const struct cpuset *cur, const struct cpuset *trial)
if (!is_cpuset_subset(trial, par))
return -EACCES;
/* If either I or some sibling (!= me) is exclusive, we can't overlap */
/*
* If either I or some sibling (!= me) is exclusive, we can't
* overlap
*/
list_for_each_entry(cont, &par->css.cgroup->children, sibling) {
c = cgroup_cs(cont);
if ((is_cpu_exclusive(trial) || is_cpu_exclusive(c)) &&
@ -744,7 +705,7 @@ static inline int started_after(void *p1, void *p2)
* @tsk: task to test
* @scan: struct cgroup_scanner contained in its struct cpuset_hotplug_scanner
*
* Call with manage_mutex held. May take callback_mutex during call.
* Call with cgroup_mutex held. May take callback_mutex during call.
* Called for each task in a cgroup by cgroup_scan_tasks().
* Return nonzero if this tasks's cpus_allowed mask should be changed (in other
* words, if its mask is not equal to its cpuset's mask).
@ -847,11 +808,11 @@ static int update_cpumask(struct cpuset *cs, char *buf)
* Temporarilly set tasks mems_allowed to target nodes of migration,
* so that the migration code can allocate pages on these nodes.
*
* Call holding manage_mutex, so our current->cpuset won't change
* during this call, as manage_mutex holds off any attach_task()
* Call holding cgroup_mutex, so current's cpuset won't change
* during this call, as cgroup_mutex holds off any attach_task()
* calls. Therefore we don't need to take task_lock around the
* call to guarantee_online_mems(), as we know no one is changing
* our tasks cpuset.
* our task's cpuset.
*
* Hold callback_mutex around the two modifications of our tasks
* mems_allowed to synchronize with cpuset_mems_allowed().
@ -896,7 +857,7 @@ static void cpuset_migrate_mm(struct mm_struct *mm, const nodemask_t *from,
* the cpuset is marked 'memory_migrate', migrate the tasks
* pages to the new memory.
*
* Call with manage_mutex held. May take callback_mutex during call.
* Call with cgroup_mutex held. May take callback_mutex during call.
* Will take tasklist_lock, scan tasklist for tasks in cpuset cs,
* lock each such tasks mm->mmap_sem, scan its vma's and rebind
* their mempolicies to the cpusets new mems_allowed.
@ -1009,7 +970,7 @@ static int update_nodemask(struct cpuset *cs, char *buf)
* tasklist_lock. Forks can happen again now - the mpol_copy()
* cpuset_being_rebound check will catch such forks, and rebind
* their vma mempolicies too. Because we still hold the global
* cpuset manage_mutex, we know that no other rebind effort will
* cgroup_mutex, we know that no other rebind effort will
* be contending for the global variable cpuset_being_rebound.
* It's ok if we rebind the same mm twice; mpol_rebind_mm()
* is idempotent. Also migrate pages in each mm to new nodes.
@ -1024,7 +985,7 @@ static int update_nodemask(struct cpuset *cs, char *buf)
mmput(mm);
}
/* We're done rebinding vma's to this cpusets new mems_allowed. */
/* We're done rebinding vmas to this cpuset's new mems_allowed. */
kfree(mmarray);
cpuset_being_rebound = NULL;
retval = 0;
@ -1038,7 +999,7 @@ int current_cpuset_is_being_rebound(void)
}
/*
* Call with manage_mutex held.
* Call with cgroup_mutex held.
*/
static int update_memory_pressure_enabled(struct cpuset *cs, char *buf)
@ -1059,7 +1020,7 @@ static int update_memory_pressure_enabled(struct cpuset *cs, char *buf)
* cs: the cpuset to update
* buf: the buffer where we read the 0 or 1
*
* Call with manage_mutex held.
* Call with cgroup_mutex held.
*/
static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs, char *buf)
@ -1193,6 +1154,7 @@ static int fmeter_getrate(struct fmeter *fmp)
return val;
}
/* Called by cgroups to determine if a cpuset is usable; cgroup_mutex held */
static int cpuset_can_attach(struct cgroup_subsys *ss,
struct cgroup *cont, struct task_struct *tsk)
{
@ -1540,7 +1502,8 @@ static int cpuset_populate(struct cgroup_subsys *ss, struct cgroup *cont)
* If this becomes a problem for some users who wish to
* allow that scenario, then cpuset_post_clone() could be
* changed to grant parent->cpus_allowed-sibling_cpus_exclusive
* (and likewise for mems) to the new cgroup.
* (and likewise for mems) to the new cgroup. Called with cgroup_mutex
* held.
*/
static void cpuset_post_clone(struct cgroup_subsys *ss,
struct cgroup *cgroup)
@ -1564,11 +1527,8 @@ static void cpuset_post_clone(struct cgroup_subsys *ss,
/*
* cpuset_create - create a cpuset
* parent: cpuset that will be parent of the new cpuset.
* name: name of the new cpuset. Will be strcpy'ed.
* mode: mode to set on new inode
*
* Must be called with the mutex on the parent inode held
* ss: cpuset cgroup subsystem
* cont: control group that the new cpuset will be part of
*/
static struct cgroup_subsys_state *cpuset_create(
@ -1769,7 +1729,7 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
* member tasks or cpuset descendants and cpus and memory, before it can
* be a candidate for release.
*
* Called with manage_mutex held. We take callback_mutex to modify
* Called with cgroup_mutex held. We take callback_mutex to modify
* cpus_allowed and mems_allowed.
*
* This walk processes the tree from top to bottom, completing one layer
@ -1910,7 +1870,7 @@ cpumask_t cpuset_cpus_allowed(struct task_struct *tsk)
/**
* cpuset_cpus_allowed_locked - return cpus_allowed mask from a tasks cpuset.
* Must be called with callback_mutex held.
* Must be called with callback_mutex held.
**/
cpumask_t cpuset_cpus_allowed_locked(struct task_struct *tsk)
{
@ -2247,10 +2207,8 @@ void __cpuset_memory_pressure_bump(void)
* - Used for /proc/<pid>/cpuset.
* - No need to task_lock(tsk) on this tsk->cpuset reference, as it
* doesn't really matter if tsk->cpuset changes after we read it,
* and we take manage_mutex, keeping attach_task() from changing it
* anyway. No need to check that tsk->cpuset != NULL, thanks to
* the_top_cpuset_hack in cpuset_exit(), which sets an exiting tasks
* cpuset to top_cpuset.
* and we take cgroup_mutex, keeping attach_task() from changing it
* anyway.
*/
static int proc_cpuset_show(struct seq_file *m, void *unused_v)
{