From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, tglx@linutronix.de,
linux-kernel@vger.kernel.org, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
frederic@kernel.org
Subject: Re: [PATCH 1/6] sched: Fix ttwu_queue_cond()
Date: Mon, 15 Jun 2020 09:45:41 -0700 [thread overview]
Message-ID: <20200615164541.GH2723@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200615133409.GS2531@hirez.programming.kicks-ass.net>
On Mon, Jun 15, 2020 at 03:34:09PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 15, 2020 at 02:56:55PM +0200, Peter Zijlstra wrote:
> > Where the condition:
> >
> > !cpus_share_cache(smp_processor_id(), cpu)
> >
> > already implies 'cpu != smp_processor_id()', because a CPU always
> > shares cache with itself, the secondary condition added in commit:
> >
> > 2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling")
> >
> > voids that implication, resulting in attempting to do local wake-ups
> > through the queue mechanism.
> >
> > Fixes: 2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling")
> > Reported-by: Paul E. McKenney <paulmck@kernel.org>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Tested-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> > kernel/sched/core.c | 13 ++++++++++++-
> > 1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2356,11 +2356,22 @@ bool cpus_share_cache(int this_cpu, int
> >
> > static inline bool ttwu_queue_cond(int cpu, int wake_flags)
> > {
> > + int this_cpu = smp_processor_id();
> > +
> > + /*
> > + * Only ever queue for remote wakeups. The on_cpu case can only ever
> > + * happen remotely, and for the normal case it makes no sense to
>
> The 'funny' thing here is, that this must be false for this patch to
> make any difference.. I just cannot see how.
>
> Also, if this is false, and p->on_cpu == 1 and p->cpu == this_cpu, then
> p _should_ be current, in which case we should never get here either,
> due to the 'p == current' special case in try_to_wake_up().
>
> The only other option is that 'p == next', but then we'd be doing
> wakeups from the middle of __schedule() and seems 'unlikely' too, esp.
> so since none of the actual stack-traces we have shows that.
>
> So colour me terribly confused.
I am rerunning with your patch 2 on the last bisection point that
resulted in scheduler NULL dereferences despite having your patch.
Hopefully some illumination will result...
Thanx, Paul
> > + * involve IPIs here, and would be broken, as many architectures cannot
> > + * trivially IPI self in any case.
> > + */
> > + if (cpu == this_cpu)
> > + return false;
next prev parent reply other threads:[~2020-06-15 16:45 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-15 12:56 [PATCH 0/6] sched: TTWU, IPI, and assorted stuff Peter Zijlstra
2020-06-15 12:56 ` [PATCH 1/6] sched: Fix ttwu_queue_cond() Peter Zijlstra
2020-06-15 13:34 ` Peter Zijlstra
2020-06-15 16:45 ` Paul E. McKenney [this message]
2020-06-15 22:58 ` Paul E. McKenney
2020-06-22 9:11 ` Mel Gorman
2020-06-22 9:41 ` Peter Zijlstra
2020-06-15 12:56 ` [PATCH 2/6] sched: Verify some SMP assumptions Peter Zijlstra
2020-06-15 12:56 ` [PATCH 3/6] sched: s/WF_ON_RQ/WQ_ON_CPU/ Peter Zijlstra
2020-06-22 9:13 ` Mel Gorman
2020-06-15 12:56 ` [PATCH 4/6] smp, irq_work: Continue smp_call_function*() and irq_work*() integration Peter Zijlstra
2020-06-15 12:56 ` [PATCH 5/6] irq_work: Cleanup Peter Zijlstra
2020-06-16 15:16 ` Petr Mladek
2020-06-15 12:57 ` [PATCH 6/6] smp: Cleanup smp_call_function*() Peter Zijlstra
2020-06-15 14:34 ` Jens Axboe
2020-06-15 16:04 ` Daniel Thompson
2020-06-17 8:23 ` Christoph Hellwig
2020-06-17 9:00 ` Peter Zijlstra
2020-06-17 11:04 ` Peter Zijlstra
2020-06-18 6:51 ` Christoph Hellwig
2020-06-18 16:25 ` Peter Zijlstra
2020-06-15 16:23 ` [PATCH 0/6] sched: TTWU, IPI, and assorted stuff Paul E. McKenney
2020-06-15 16:40 ` Peter Zijlstra
2020-06-15 17:21 ` Paul E. McKenney
2020-06-15 19:11 ` Peter Zijlstra
2020-06-15 19:55 ` Paul E. McKenney
2020-06-16 16:31 ` Paul E. McKenney
2020-06-16 17:04 ` Peter Zijlstra
2020-06-16 17:17 ` Peter Zijlstra
2020-06-16 17:53 ` Paul E. McKenney
2020-06-19 13:44 ` Peter Zijlstra
2020-06-19 17:20 ` Paul E. McKenney
2020-06-19 17:48 ` Paul E. McKenney
2020-06-19 18:11 ` Peter Zijlstra
2020-06-19 18:46 ` Paul E. McKenney
2020-06-20 18:46 ` Paul E. McKenney
2020-06-16 17:51 ` Paul E. McKenney
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