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 <[email protected]>
> > Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
> > Tested-by: Paul E. McKenney <[email protected]>
> > ---
> > 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;
