linux-kernel - Re: [PATCH 2/6] sched,trace: Add a tracepoint for remote wakeups via polling

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Date:	Wed, 4 Jun 2014 07:43:42 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, nicolas.pitre@...aro.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] sched,trace: Add a tracepoint for remote wakeups via polling

On Wed, Jun 4, 2014 at 6:00 AM, Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
> On 06/04/2014 02:29 AM, Andy Lutomirski wrote:
>>
>> Remote wakeups of polling CPUs are a valuable performance
>> improvement; add a tracepoint to make it much easier to verify that
>> they're working.
>>
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>
>
> I don't think this trace makes sense. The polling state is x86 only and this
> trace is in the generic code.
>
> Furthermore, you may be not in polling state but in the idle mainloop before
> or after the idle state, so the trace will be wrong for the purpose you are
> aiming.
>
> IMO, the right place would be in 'poll_idle' but why add such trace ?
>
> If, on x86, we exit poll_idle, we have the idle state exit trace. The
> missing information would be the origin of the 'wakeup' (irq or ipi or
> nothing). The missing informations are the IPI traces [1].
>
> And as a sidenote, the polling state could be rare on a system with a
> cpuidle driver, it should be much more easy to restrict the idle states to
> 'poll' only and check there are no IPI_WAKEUP, no ?

While developing and testing this, I found it quite useful to measure
the ratio of IPIs to non-IPIs.  I think most of your objection stems
from the fact that I named this tracepoint rather poorly.  How about
"sched_wake_idle_without_ipi"?  I don't think there's anywhere good to
put it in arch / cpuidle driver code (neither the true polling loop
nor (AFAIK) x86 mwait the sleeping cpu any straightforward indication
of why it woke up.

On archs without TIF_POLLING_NRFLAG, the compiler should optimize this
away completely.  On archs with TIF_POLLING_NRFLAG but not actual
polling, it could be interesting to see if this tracepoint ever hits
and, if not, consider removing TIF_POLLING_NRFLAG.

--Andy
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