On Tue, Feb 01, 2022 at 12:37:27PM +0100, Jan Beulich wrote:
> On 01.02.2022 12:04, Roger Pau Monné wrote:
> > 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 <jbeul...@suse.com>
> >> ---
> >> 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.
>
> Okay, I guess I'll to the larger rework then.
>
> >> --- 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 did consider it but then decided against. Even leaving this aspect
> aside the counter only counts _attempts_ to enter a certain state;
> the CPU may find reasons to never actually enter it. And what we have
> when before == after is still an attempt, albeit an unsuccessful one.
Right, in which case:
Acked-by: Roger Pau Monné <roger....@citrix.com>
Not sure whether you would like to commit this now and do the lager
rework as a followup patch. That would be fine by me.
Thanks, Roger.
