linux-kernel - Re: [PATCH] Force processes to non-realtime before mm

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Message-ID: <alpine.DEB.2.20.1609021700530.5647@nanos>
Date:   Fri, 2 Sep 2016 17:02:29 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Brian Silverman <brian@...oton-tech.com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        bigeasy@...utronix.de, Ingo Molnar <mingo@...nel.org>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH] Force processes to non-realtime before mm_exit

On Thu, 14 Jul 2016, Peter Zijlstra wrote:
> On Fri, Jun 03, 2016 at 04:18:44PM -0700, Brian Silverman wrote:
> > Without this, a realtime process which has called mlockall exiting
> > causes large latencies for other realtime processes at the same or
> > lower priorities. This seems like a fairly common use case too, because
> > realtime processes generally want their memory locked into RAM.
> 
> So I'm not too sure..  SCHED_FIFO/RR are a complete trainwreck and
> provide absolutely no isolation from badly behaving tasks what so ever,
> so I'm not too inclined to protect them from exit either, its just one
> more way in which they can cause pain.
> 
> But aside from the, the patch has issues..
> 
> > +++ b/kernel/exit.c
> > @@ -730,6 +730,12 @@ void do_exit(long code)
> >  	tsk->exit_code = code;
> >  	taskstats_exit(tsk, group_dead);
> >  
> > +	if (tsk->policy == SCHED_FIFO || tsk->policy == SCHED_RR) {
> > +		struct sched_param param = { .sched_priority = 0 };
> > +
> > +		sched_setscheduler_nocheck(current, SCHED_NORMAL, &param);
> > +	}
> > +
> >  	exit_mm(tsk);
> 
> That only does half a job. You forget about SCHED_DEADLINE and negative
> nice tasks.
> 
> Something like the below perhaps... But yeah, unconvinced.

I agree that FIFO/RR can cause pain, but running exit_mm() with RT priority
or consuming DL time is silly.
 
FWIW: Acked-by-me