Re: [PATCH v3 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

On Tue 01-09-20 18:25:58, Suren Baghdasaryan wrote:

Currently __set_oom_adj loops through all processes in the system to keep oom_score_adj and oom_score_adj_min in sync between processes sharing their mm. This is done for any task with more that one mm_users, which includes processes with multiple threads (sharing mm and signals). However for such processes the loop is unnecessary because their signal structure is shared as well. Android updates oom_score_adj whenever a tasks changes its role (background/foreground/...) or binds to/unbinds from a service, making it more/less important. Such operation can happen frequently. We noticed that updates to oom_score_adj became more expensive and after further investigation found out that the patch mentioned in "Fixes" introduced a regression. Using Pixel 4 with a typical Android workload, write time to oom_score_adj increased from ~3.57us to ~362us. Moreover this regression linearly depends on the number of multi-threaded processes running on the system. Mark the mm with a new MMF_MULTIPROCESS flag bit when task is created with (CLONE_VM && !CLONE_THREAD && !CLONE_VFORK). Change __set_oom_adj to use MMF_MULTIPROCESS instead of mm_users to decide whether oom_score_adj update should be synchronized between multiple processes. To prevent races between clone() and __set_oom_adj(), when oom_score_adj of the process being cloned might be modified from userspace, we use oom_adj_mutex. Its scope is changed to global. The combination of (CLONE_VM && !CLONE_THREAD) is rarely used except for the case of vfork(). To prevent performance regressions of vfork(), we skip taking oom_adj_mutex and setting MMF_MULTIPROCESS when CLONE_VFORK is specified. Clearing the MMF_MULTIPROCESS flag (when the last process sharing the mm exits) is left out of this patch to keep it simple and because it is believed that this threading model is rare. Should there ever be a need for optimizing that case as well, it can be done by hooking into the exit path, likely following the mm_update_next_owner pattern. With the combination of (CLONE_VM && !CLONE_THREAD && !CLONE_VFORK) being quite rare, the regression is gone after the change is applied.

Fixes: 44a70adec910 ("mm, oom_adj: make sure processes sharing mm have same view of oom_score_adj") Reported-by: Tim Murray timmurray@google.com Debugged-by: Minchan Kim minchan@kernel.org Suggested-by: Michal Hocko mhocko@kernel.org Signed-off-by: Suren Baghdasaryan surenb@google.com

Acked-by: Michal Hocko mhocko@suse.com

---

v3:

• Addressed Eric Biederman's comments from:

https://lore.kernel.org/linux-mm/87imd6n0qk.fsf@x220.int.ebiederm.org/ -- renabled oom_adj_lock back to oom_adj_mutex -- renamed MMF_PROC_SHARED into MMF_MULTIPROCESS and fixed its comment

• Updated description to reflect the change

v2:

• https://lore.kernel.org/linux-mm/20200824153036.3201505-1-surenb@google.com/
• Implemented proposal from Michal Hocko in:

https://lore.kernel.org/linux-fsdevel/20200820124109.GI5033@dhcp22.suse.cz/

• Updated description to reflect the change

v1:

• https://lore.kernel.org/linux-mm/20200820002053.1424000-1-surenb@google.com/

fs/proc/base.c | 3 +-- include/linux/oom.h | 1 + include/linux/sched/coredump.h | 1 + kernel/fork.c | 21 +++++++++++++++++++++ mm/oom_kill.c | 2 ++ 5 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c index 617db4e0faa0..aa69c35d904c 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -1055,7 +1055,6 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count, static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) {

• static DEFINE_MUTEX(oom_adj_mutex); struct mm_struct *mm = NULL; struct task_struct *task; int err = 0;

@@ -1095,7 +1094,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) struct task_struct *p = find_lock_task_mm(task); if (p) {

• 	if (atomic_read(&p->mm->mm_users) > 1) {
  

• 	if (test_bit(MMF_MULTIPROCESS, &p->mm->flags)) {
  	mm = p->mm;
  	mmgrab(mm);
  }
  

diff --git a/include/linux/oom.h b/include/linux/oom.h index f022f581ac29..2db9a1432511 100644 --- a/include/linux/oom.h +++ b/include/linux/oom.h @@ -55,6 +55,7 @@ struct oom_control { }; extern struct mutex oom_lock; +extern struct mutex oom_adj_mutex; static inline void set_current_oom_origin(void) { diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h index ecdc6542070f..dfd82eab2902 100644 --- a/include/linux/sched/coredump.h +++ b/include/linux/sched/coredump.h @@ -72,6 +72,7 @@ static inline int get_dumpable(struct mm_struct *mm) #define MMF_DISABLE_THP 24 /* disable THP for all VMAs */ #define MMF_OOM_VICTIM 25 /* mm is the oom victim */ #define MMF_OOM_REAP_QUEUED 26 /* mm was queued for oom_reaper */ +#define MMF_MULTIPROCESS 27 /* mm is shared between processes */ #define MMF_DISABLE_THP_MASK (1 << MMF_DISABLE_THP) #define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\ diff --git a/kernel/fork.c b/kernel/fork.c index 4d32190861bd..6129a88c19ad 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1809,6 +1809,25 @@ static __always_inline void delayed_free_task(struct task_struct *tsk) free_task(tsk); } +static void copy_oom_score_adj(u64 clone_flags, struct task_struct *tsk) +{

• /* Skip if kernel thread */
• if (!tsk->mm)

• return;
  

• /* Skip if spawning a thread or using vfork */
• if ((clone_flags & (CLONE_VM | CLONE_THREAD | CLONE_VFORK)) != CLONE_VM)

• return;
  

• /* We need to synchronize with __set_oom_adj */
• mutex_lock(&oom_adj_mutex);
• set_bit(MMF_MULTIPROCESS, &tsk->mm->flags);
• /* Update the values in case they were changed after copy_signal */
• tsk->signal->oom_score_adj = current->signal->oom_score_adj;
• tsk->signal->oom_score_adj_min = current->signal->oom_score_adj_min;
• mutex_unlock(&oom_adj_mutex);

+}

/*

• This creates a new process as a copy of the old one,
• but does not actually start it yet.

@@ -2281,6 +2300,8 @@ static __latent_entropy struct task_struct *copy_process( trace_task_newtask(p, clone_flags); uprobe_copy_process(p, clone_flags);

• copy_oom_score_adj(clone_flags, p);
• return p;

bad_fork_cancel_cgroup: diff --git a/mm/oom_kill.c b/mm/oom_kill.c index e90f25d6385d..8b84661a6410 100644 --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -64,6 +64,8 @@ int sysctl_oom_dump_tasks = 1;

• and mark_oom_victim

*/ DEFINE_MUTEX(oom_lock); +/* Serializes oom_score_adj and oom_score_adj_min updates */ +DEFINE_MUTEX(oom_adj_mutex); static inline bool is_memcg_oom(struct oom_control *oc) { -- 2.28.0.526.ge36021eeef-goog