On Thu, Jan 27, 2022 at 04:13:47PM +0100, Jan Beulich wrote:
> While we don't want to skip calling update_idle_stats(), arrange for it
> to not increment the overall time spent in the state we didn't really
> enter.
>
> Signed-off-by: Jan Beulich <[email protected]>
> ---
> RFC: If we wanted to also move the tracing, then I think the part ahead
> of the if() also would need moving. At that point we could as well
> move update_last_cx_stat(), too, which afaict would allow skipping
> update_idle_stats() on the "else" path (which therefore would go
> away). Yet then, with the setting of power->safe_state moved up a
> little (which imo it should have been anyway) the two
> cpu_is_haltable() invocations would only have the lapic_timer_off()
> invocation left in between. This would then seem to call for simply
> ditching the 2nd one - acpi-idle also doesn't have a 2nd instance.
It's possible for lapic_timer_off to take a non-trivial amount of time
when virtualized, but it's likely we won't be using mwait in that
case, so not sure it matter much to have the two cpu_is_haltable calls
if there's just a lapic_timer_off between them.
> TBD: For the tracing I wonder if that really needs to come ahead of the
> local_irq_enable(). Maybe trace_exit_reason() needs to, but quite
> certainly TRACE_6D() doesn't.
Would be good if it could be moved after the local_irq_enable call, as
it's not as trivial as I've expected, and will just add latency to any
pending interrupt waiting to be serviced. FWIW, I haven't spotted a
need to call it with interrupt disabled.
> ---
> v3: Also move cstate_restore_tsc() invocation and split ones to
> update_idle_stats().
> v2: New.
>
> --- a/xen/arch/x86/cpu/mwait-idle.c
> +++ b/xen/arch/x86/cpu/mwait-idle.c
> @@ -854,17 +854,23 @@ static void mwait_idle(void)
> mwait_idle_with_hints(cx->address, MWAIT_ECX_INTERRUPT_BREAK);
>
> local_irq_disable();
> - }
>
> - after = alternative_call(cpuidle_get_tick);
> + after = alternative_call(cpuidle_get_tick);
> +
> + cstate_restore_tsc();
> +
> + /* Now back in C0. */
> + update_idle_stats(power, cx, before, after);
> + } else {
> + /* Never left C0. */
> + after = alternative_call(cpuidle_get_tick);
> + update_idle_stats(power, cx, after, after);
While adjusting this, could you also modify update_idle_stats to avoid
increasing cx->usage if before == after (or !sleep_ticks). I don't
think it's fine to increase the state counter if we never actually
entered it.
I was also going to suggest that you don't set 'after' and just use
update_idle_stats(power, cx, before, before); but seeing as TRACE_6D
also makes use of 'after' there's not much point without further
rework. I also see the RFC note at the top, so while I think this is
an improvement, I agree it would be nice to avoid the trace altogether
if we never actually enter the state. If you want to rework the patch
or send a followup that's fine for me.
Thanks, Roger.
