Re: [PATCH 1/6] sched: Fix ttwu_queue_cond()

[Peter Zijlstra <peterz@infradead.org>]

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;