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

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Date:   Thu, 20 Aug 2020 13:41:53 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        christian.brauner@...ntu.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 08/20, Michal Hocko wrote:
>
> On Thu 20-08-20 13:13:55, Michal Hocko wrote:
> > On Thu 20-08-20 12:55:56, Oleg Nesterov wrote:
> > > On 08/19, Suren Baghdasaryan wrote:
> > > >
> > > > 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.
> > >
> > > vfork() ?
> >
> > Could you be more specific?

I meant, vfork() is not rare and iiuc MMF_PROC_SHARED will be set in this
case and never cleared.

> > > > --- 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)) {
> > >
> > > I agree with Christian, you need CLONE_THREAD
> >
> > This was my suggestion to Suren, likely because I've misrememberd which
> > clone flag is responsible for the signal delivery. But now, after double
> > checking we do explicitly disallow CLONE_SIGHAND && !CLONE_VM. So
> > CLONE_THREAD is the right thing to check.
>
> I have tried to remember but I have to say that after reading man page I
> am still confused. So what is the actual difference between CLONE_THREAD
> and CLONE_SIGHAND?

Well, CLONE_THREAD creates a sub-thred, it needs CLONE_SIGHAND/VM.

> Essentially all we care about from the OOM (and
> oom_score_adj) POV is that signals are delivered to all entities and
> that thay share signal struct.

Yes, but CLONE_SIGHAND doesn't share the signal struct.

CLONE_SIGHAND shares sighand_struct, iow it shares the signal handlers.
so that if (say) the child does signal(SIG, handler) this equally affects
the parent. This obviously means that CLONE_SIGHAND needs CLONE_VM.

Oleg.