From: Quentin Perret <qperret@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>,
linux-kernel@vger.kernel.org, aaron.lwe@gmail.com,
valentin.schneider@arm.com, mingo@kernel.org, pauld@redhat.com,
jdesfossez@digitalocean.com, naravamudan@digitalocean.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
juri.lelli@redhat.com, rostedt@goodmis.org, bsegall@google.com,
mgorman@suse.de, kernel-team@android.com, john.stultz@linaro.org
Subject: Re: NULL pointer dereference in pick_next_task_fair
Date: Fri, 8 Nov 2019 12:15:26 +0000 [thread overview]
Message-ID: <20191108121526.GB83597@google.com> (raw)
In-Reply-To: <20191108120034.GK4131@hirez.programming.kicks-ass.net>
On Friday 08 Nov 2019 at 13:00:35 (+0100), Peter Zijlstra wrote:
> On Fri, Nov 08, 2019 at 11:02:12AM +0000, Quentin Perret wrote:
> > On Thursday 07 Nov 2019 at 20:29:07 (+0100), Peter Zijlstra wrote:
> > > I still havne't had food, but this here compiles...
> >
> > And it seems to work, too :)
>
> Excellent!
>
> > > @@ -3929,13 +3929,17 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > > }
> > >
> > > restart:
> > > - /*
> > > - * Ensure that we put DL/RT tasks before the pick loop, such that they
> > > - * can PULL higher prio tasks when we lower the RQ 'priority'.
> > > - */
> > > - prev->sched_class->put_prev_task(rq, prev, rf);
> > > - if (!rq->nr_running)
> > > - newidle_balance(rq, rf);
> > > +#ifdef CONFIG_SMP
> > > + for (class = prev->sched_class;
> > > + class != &idle_sched_class;
> > > + class = class->next) {
> > > +
> > > + if (class->balance(rq, prev, rf))
> > > + break;
> > > + }
> > > +#endif
> > > +
> > > + put_prev_task(rq, prev);
> >
> > Right, that looks much cleaner IMO. I'm thinking if we killed the
> > special case for CFS above we could do with a single loop to iterate the
> > classes, and you could fold ->balance() in ->pick_next_task() ...
>
> No, you can't, because then you're back to having to restart the pick
> when something happens when we drop the rq halfway down the pick. Which
> is something I really wanted to get rid of.
Right, with a single loop you'll have to re-iterate the classes from
the start in case of RETRY_TASK, but you're re-iterating all the classes
too with this patch. You're doing a little less work in the second loop
though, so maybe it's worth it. And I was the one worried about
refactoring the code too much close to the release, so maybe that's for
another time ;)
Thanks,
Quentin
next prev parent reply other threads:[~2019-11-08 12:15 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-28 17:46 NULL pointer dereference in pick_next_task_fair Quentin Perret
2019-10-28 21:49 ` Peter Zijlstra
2019-10-29 11:34 ` Peter Zijlstra
2019-10-29 11:50 ` Quentin Perret
2019-10-30 22:50 ` Ram Muthiah
2019-10-31 1:33 ` Valentin Schneider
2019-10-31 10:54 ` Valentin Schneider
2019-10-31 14:24 ` Valentin Schneider
2019-10-31 22:15 ` Valentin Schneider
2019-11-06 12:05 ` Peter Zijlstra
2019-11-06 13:08 ` Peter Zijlstra
2019-11-06 15:04 ` Qais Yousef
2019-11-06 16:57 ` Peter Zijlstra
2019-11-06 17:26 ` Qais Yousef
2019-11-06 15:51 ` Kirill Tkhai
2019-11-06 16:54 ` Peter Zijlstra
2019-11-06 17:27 ` Peter Zijlstra
2019-11-07 8:36 ` Kirill Tkhai
2019-11-07 13:26 ` Peter Zijlstra
2019-11-07 15:12 ` Kirill Tkhai
2019-11-07 15:42 ` Peter Zijlstra
2019-11-07 15:53 ` Kirill Tkhai
2019-11-07 15:38 ` Quentin Perret
2019-11-07 18:43 ` Peter Zijlstra
2019-11-07 19:27 ` Quentin Perret
2019-11-07 19:31 ` Peter Zijlstra
2019-11-07 19:42 ` Quentin Perret
2019-11-07 19:29 ` Peter Zijlstra
2019-11-08 11:02 ` Quentin Perret
2019-11-08 11:47 ` Valentin Schneider
2019-11-08 11:58 ` Quentin Perret
2019-11-08 12:00 ` Peter Zijlstra
2019-11-08 12:15 ` Quentin Perret [this message]
2019-11-08 12:35 ` Peter Zijlstra
2019-11-08 12:24 ` Peter Zijlstra
2019-11-08 11:55 ` Peter Zijlstra
2019-11-08 12:52 ` Peter Zijlstra
2019-11-07 16:09 ` Qais Yousef
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