diff options
| -rw-r--r-- | kernel/cpuset.c | 128 |
1 files changed, 43 insertions, 85 deletions
diff --git a/kernel/cpuset.c b/kernel/cpuset.c index 20cb391..918bee9 100644 --- a/kernel/cpuset.c +++ b/kernel/cpuset.c @@ -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) { |
