linux-kernel - Re: [PATCHSET v6 0/4] Split iowait into two states

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Message-ID: <20240905093607.GB15400@noisy.programming.kicks-ass.net>
Date: Thu, 5 Sep 2024 11:36:07 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, daniel.lezcano@...aro.org,
	linux-pm@...r.kernel.org,
	Christian Loehle <christian.loehle@....com>
Subject: Re: [PATCHSET v6 0/4] Split iowait into two states

On Wed, Sep 04, 2024 at 05:18:57PM +0200, Rafael J. Wysocki wrote:

> To be more precise, there are two different uses of "iowait" in PM.
> 
> One is the nr_iowait_cpu() call in menu_select() and the result of it
> is used for two purposes: (1) select different sets of statistics
> depending on whether or not this number is zero and (2) set a limit
> for the idle state's exit latency that depends on this number (but
> note that it only takes effect when the "iowait" statistics are used
> in the first place).  Both of these are arguably questionable and it
> is unclear to me whether or not they actually help and how much.

So this one is very dubious, it relies on tasks getting back on the CPU
they went to sleep on -- not guaranteed at all.

> The other use is boosting CPU frequency in schedutil and intel_pstate
> if SCHED_CPUFREQ_IOWAIT is passed to them which in turn depends on the
> p->in_iowait value in enqueue_task_fair().

This one is fine and makes sense. At this point we know that p is going
to run and where it is going to run.