linux-kernel - Re: [PATCH RFC] cpuidle: consolidate calls to time capture

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Date:   Wed, 18 Mar 2020 12:04:52 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     ulf.hansson@...aro.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, khilman@...nel.org
Subject: Re: [PATCH RFC] cpuidle: consolidate calls to time capture

Hi Rafael,

On 18/03/2020 11:17, Rafael J. Wysocki wrote:
> On Monday, March 16, 2020 10:08:43 PM CET Daniel Lezcano wrote:
>> A few years ago, we changed the code in cpuidle to replace ktime_get()
>> by a local_clock() to get rid of potential seq lock in the path and an
>> extra latency.
>>
>> Meanwhile, the code evolved and we are getting the time in some other
>> places like the power domain governor and in the future break even
>> deadline proposal.
> 
> Hmm?
> 
> Have any patches been posted for that?

https://lkml.org/lkml/2020/3/11/1113

https://lkml.org/lkml/2020/3/13/466

but there is no consensus yet if that has a benefit or not.

>> Unfortunately, as the time must be compared across the CPU, we have no
>> other option than using the ktime_get() again. Hopefully, we can
>> factor out all the calls to local_clock() and ktime_get() into a
>> single one when the CPU is entering idle as the value will be reuse in
>> different places.
> 
> So there are cases in which it is not necessary to synchronize the time
> between CPUs and those would take the overhead unnecessarily.
> 
> This change looks premature to me at least.

The idea is to call one time ktime_get() when entering idle and store
the result in the struct cpuidle_device, so we have the information when
we entered idle.

Moreover, ktime_get() is called in do_idle() via:

tick_nohz_idle_enter()
  tick_nohz_start_idle()
    ts->idle_entrytime = ktime_get();

This is called at the first loop level. The idle loop is exiting and
re-entering again without passing through tick_nohz_idle_enter() in the
second loop level in case of interrupt processing, thus the
idle_entrytime is not updated and the return of
tick_nohz_get_sleep_length() will be greater than what is expected.

May be we can consider ktime_get_mono_fast_ns() which is lockless with a
particular care of the non-monotonic aspect if needed. Given the
description at [1] the time jump could a few nanoseconds in case of NMI.

The local_clock() can no be inspected across CPUs, the gap is too big
and continues to increase during system lifetime.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/time/timekeeping.c#n396

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