Re: [RFC][PATCH] x86: optimization to avoid CAL+RES IPIs

[Josh Don <joshdon@google.com>]

Hey Andy, thanks for taking a look.

On Fri, Jul 17, 2020 at 8:14 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> PeterZ and I fixed a whole series of bugs a few years ago, and remote
> wakeups *should* already do this. Did we miss something?  Did it
> regress?  Even the call_function_single path ought to go through this:
>
> void send_call_function_single_ipi(int cpu)
> {
>         struct rq *rq = cpu_rq(cpu);
>
>         if (!set_nr_if_polling(rq->idle))
>                 arch_send_call_function_single_ipi(cpu);
>         else
>                 trace_sched_wake_idle_without_ipi(cpu);
> }
>

Yep, I was sitting on this for a bit and raced with b2a02fc43a there.
90b5363ac also got rid of the last smp_send_reschedule() that was
triggering the ipiless handling.

One of the nice parts of the patch was that it could blanket apply to
all of the smp_call/reschedule.  However, with the above patches that
isn't a concern; it makes more sense to keep the existing
TIF_POLLING_NRFLAG logic.