linux-kernel - RE: [PATCH] cpuidle: poll_state: Revise loop termination condition

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Date:   Sat, 10 Nov 2018 13:50:01 -0800
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Rafael J. Wysocki'" <rjw@...ysocki.net>
Cc:     "'Peter Zijlstra'" <peterz@...radead.org>,
        "'LKML'" <linux-kernel@...r.kernel.org>,
        "'Daniel Lezcano'" <daniel.lezcano@...aro.org>,
        "'Linux PM'" <linux-pm@...r.kernel.org>
Subject: RE: [PATCH] cpuidle: poll_state: Revise loop termination condition

On 2018.10.02 14:51 Rafael J. Wysocki wrote:

> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> If need_resched() returns "false", breaking out of the loop in
> poll_idle() will cause a new idle state to be selected, so in fact
> usually it doesn't make sense to spin in it longer than the target
> residency of the second state.  [Note that the "polling" state is
> used only if there is at least one "real" state defined in addition
> to it.]  On the other hand, breaking out of it early (say in case
> the next state is disabled) shouldn't hurt as it is polling anyway.

While I agree that it is polling anyway, this change can add significant
burden when debugging and trace is enabled for cpu_idle, if idle state 0
is used often.

For example: Phoronix dbench test, 96 clients: 900 second trace:

Kernel 4.20-rc1:
idle state 0 entry exits: 686,724
Does trace being enabled effect the system under test: Yes.

Kernel 4.20-rc1 with this patch reverted:
idle state 0 entry exits: 66,185
Does trace being enabled effect the system under test: No, or minimal.

... Doug