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

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Date:   Wed, 18 Mar 2020 11:17:12 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
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

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?

> 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.

Thanks!