linux-kernel - Re: [PATCH 1/6] sched: Fix ttwu_queue

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Message-ID: <20200615225810.GA8492@paulmck-ThinkPad-P72>
Date:   Mon, 15 Jun 2020 15:58:10 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, frederic@...nel.org
Subject: Re: [PATCH 1/6] sched: Fix ttwu_queue_cond()

On Mon, Jun 15, 2020 at 09:45:41AM -0700, Paul E. McKenney wrote:
> 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@...nel.org>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > Tested-by: Paul E. McKenney <paulmck@...nel.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...

No, Mr. Murphy is out in force.  I saw only the NULL pointer
dereferences without any WARN()s.  :-/

						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;