linux-kernel - Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom

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Message-ID: <20200820124109.GI5033@dhcp22.suse.cz>
Date:   Thu, 20 Aug 2020 14:41:09 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>, mingo@...nel.org,
        peterz@...radead.org, tglx@...utronix.de, esyr@...hat.com,
        christian@...lner.me, areber@...hat.com, shakeelb@...gle.com,
        cyphar@...har.com, adobriyan@...il.com, akpm@...ux-foundation.org,
        ebiederm@...ssion.com, gladkov.alexey@...il.com, walken@...gle.com,
        daniel.m.jordan@...cle.com, avagin@...il.com,
        bernd.edlinger@...mail.de, john.johansen@...onical.com,
        laoar.shao@...il.com, timmurray@...gle.com, minchan@...nel.org,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in
 __set_oom_adj when not necessary

On Thu 20-08-20 13:42:56, Michal Hocko wrote:
> On Thu 20-08-20 13:30:23, Christian Brauner wrote:
[...]
> > trying to rely on set_bit() and test_bit() in copy_mm() being atomic and
> > then calling it where Oleg said after the point of no return.
> 
> No objections.

Would something like the following work for you?

diff --git a/kernel/fork.c b/kernel/fork.c
index 9177a76bf840..25b83f0912a6 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1403,15 +1403,6 @@ 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);
-			/* 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_lock);
-		}
 		goto good_mm;
 	}
 
@@ -1818,6 +1809,19 @@ 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)
+{
+	if ((clone_flags & (CLONE_VM | CLONE_THREAD | CLONE_VFORK)) == CLONE_VM) {
+		/* We need to synchronize with __set_oom_adj */
+		mutex_lock(&oom_adj_lock);
+		set_bit(MMF_PROC_SHARED, &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_lock);
+	}
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -2290,6 +2294,8 @@ 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:
-- 
Michal Hocko
SUSE Labs