Messages in this thread | | | Date | Mon, 15 Jun 2020 15:58:10 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/6] sched: Fix ttwu_queue_cond() |
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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@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...
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;
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