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

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Message-ID: <20200820112932.GG5033@dhcp22.suse.cz>
Date:   Thu, 20 Aug 2020 13:29:32 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Oleg Nesterov <oleg@...hat.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 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?
> 
> > > --- 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? 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. copy_signal is checking for CLONE_THREAD
but CLONE_THREAD requires CLONE_SIGHAND AFAIU. So is there any cae where
checking for CLONE_SIGHAND would wrong for our purpose?

This is mostly an academic question because I do agree that checking for
CLONE_THREAD is likely more readable. And in fact the MMF_PROC_SHARED is
likely more suitable to be set in copy_signal. 

-- 
Michal Hocko
SUSE Labs