Re: [PATCH v3 2/2] x86/mwait-idle: squash stats update when not actually entering C-state

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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 <jbeulich@xxxxxxxx>
> >> ---
> >> 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.pau@xxxxxxxxxx>

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.