From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A5734C433E1 for ; Thu, 20 Aug 2020 10:33:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6E86220658 for ; Thu, 20 Aug 2020 10:33:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730888AbgHTKdk (ORCPT ); Thu, 20 Aug 2020 06:33:40 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:38234 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731652AbgHTKdQ (ORCPT ); Thu, 20 Aug 2020 06:33:16 -0400 Received: from ip5f5af70b.dynamic.kabel-deutschland.de ([95.90.247.11] helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1k8hsH-0003ho-Hd; Thu, 20 Aug 2020 10:32:49 +0000 Date: Thu, 20 Aug 2020 12:32:48 +0200 From: Christian Brauner To: Michal Hocko Cc: Suren Baghdasaryan , mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, esyr@redhat.com, christian@kellner.me, areber@redhat.com, shakeelb@google.com, cyphar@cyphar.com, oleg@redhat.com, adobriyan@gmail.com, akpm@linux-foundation.org, ebiederm@xmission.com, gladkov.alexey@gmail.com, walken@google.com, daniel.m.jordan@oracle.com, avagin@gmail.com, bernd.edlinger@hotmail.de, john.johansen@canonical.com, laoar.shao@gmail.com, timmurray@google.com, minchan@kernel.org, kernel-team@android.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary Message-ID: <20200820103248.vkzrndewvy5vlncz@wittgenstein> References: <20200820002053.1424000-1-surenb@google.com> <20200820084654.jdl6jqgxsga7orvf@wittgenstein> <20200820090901.GD5033@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200820090901.GD5033@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 20, 2020 at 11:09:01AM +0200, Michal Hocko wrote: > On Thu 20-08-20 10:46:54, Christian Brauner wrote: > > On Wed, Aug 19, 2020 at 05:20:53PM -0700, 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_PROC_SHARED flag bit when task is created with > > > CLONE_VM and !CLONE_SIGHAND. Change __set_oom_adj to use MMF_PROC_SHARED > > > 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 and it is renamed into oom_adj_lock for naming consistency with > > > oom_lock. Since the combination of CLONE_VM and !CLONE_SIGHAND is rarely > > > used the additional mutex lock in that path of the clone() syscall should > > > not affect its overall performance. Clearing the MMF_PROC_SHARED 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 and !CLONE_SIGHAND 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 > > > Suggested-by: Michal Hocko > > > Signed-off-by: Suren Baghdasaryan > > > --- > > > fs/proc/base.c | 7 +++---- > > > include/linux/oom.h | 1 + > > > include/linux/sched/coredump.h | 1 + > > > kernel/fork.c | 9 +++++++++ > > > mm/oom_kill.c | 2 ++ > > > 5 files changed, 16 insertions(+), 4 deletions(-) > > > > > > diff --git a/fs/proc/base.c b/fs/proc/base.c > > > index 617db4e0faa0..cff1a58a236c 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; > > > @@ -1064,7 +1063,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) > > > if (!task) > > > return -ESRCH; > > > > > > - mutex_lock(&oom_adj_mutex); > > > + mutex_lock(&oom_adj_lock); > > > if (legacy) { > > > if (oom_adj < task->signal->oom_score_adj && > > > !capable(CAP_SYS_RESOURCE)) { > > > @@ -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_PROC_SHARED, &p->mm->flags)) { > > > mm = p->mm; > > > mmgrab(mm); > > > } > > > @@ -1132,7 +1131,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy) > > > mmdrop(mm); > > > } > > > err_unlock: > > > - mutex_unlock(&oom_adj_mutex); > > > + mutex_unlock(&oom_adj_lock); > > > put_task_struct(task); > > > return err; > > > } > > > diff --git a/include/linux/oom.h b/include/linux/oom.h > > > index f022f581ac29..861f22bd4706 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_lock; > > > > > > static inline void set_current_oom_origin(void) > > > { > > > diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h > > > index ecdc6542070f..070629b722df 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_PROC_SHARED 27 /* mm is shared while sighand is not */ > > > #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..9177a76bf840 100644 > > > --- a/kernel/fork.c > > > +++ b/kernel/fork.c > > > @@ -1403,6 +1403,15 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk) > > > if (clone_flags & CLONE_VM) { > > > mmget(oldmm); > > > mm = oldmm; > > > + if (!(clone_flags & CLONE_SIGHAND)) { > > > + /* We need to synchronize with __set_oom_adj */ > > > + mutex_lock(&oom_adj_lock); > > > + set_bit(MMF_PROC_SHARED, &mm->flags); > > > > This seems fine. > > > > > + /* 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; > > > > But this seems wrong to me. > > copy_signal() should be the only place where ->signal is set. Just from > > a pure conceptual perspective. The copy_*() should be as self-contained > > as possible imho. > > Also, now I have to remember/look for two different locations where > > oom_score_adj{_min} is initialized during fork. And this also creates a > > dependency between copy_signal() and copy_mm() that doesn't need to be > > there imho. I'm not a fan. > > Yes, this is not great but we will need it because the __set_oom_adj > might happen between copy_signal and copy_mm. If that happens then > __set_oom_adj misses this newly created task and so it will have a > disagreeing oom_score_adj. One more clarification. The commit message states that > > > 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. and it seems you want to exclude threads, i.e. only update mm that is shared not among threads in the same thread-group. But struct signal and struct sighand_struct are different things, i.e. they can be shared or not independent of each other. A non-shared signal_struct where oom_score_adj{_min} live is only implied by !CLONE_THREAD. So shouldn't this be: if (!(clone_flags & CLONE_THREAD)) rather than CLONE_SIGHAND? I might be missing something obvious though... Christian