Messages in this thread | | | Date | Mon, 15 Jun 2020 15:34:09 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/6] sched: Fix ttwu_queue_cond() |
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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.
> + * 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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