linux-kernel - Re: [PATCH] ACPI: Remove msleep() bloat from acpi_os

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Message-ID: <87ikvjah67.fsf@somnus>
Date: Thu, 29 Aug 2024 10:04:32 +0200
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Len Brown <lenb@...nel.org>, "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-acpi@...r.kernel.org,
 linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, Frederic
 Weisbecker <frederic@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Jonathan Corbet <corbet@....net>, Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ACPI: Remove msleep() bloat from acpi_os_sleep()

Anna-Maria Behnsen <anna-maria@...utronix.de> writes:

[...]

> Lets have a deeper look to msleep() internals: msleep() uses timer list
> timers. Because of the design of the timer wheel (granularity of buckets
> increases with the levels) and because of the granularity of jiffies the
> sleep values will be longer as specified. Let's assume we are executing
> a msleep(1) on a HZ=250 system:
>
> First msecs are mapped on jiffies, so this results in a 4ms timeout
> value, as there is nothing shorter than 1 jiffie. Then the jiffie value
> is handed over to timer code and msleep() adds another jiffie to the
> timeout. The timeout is then 2 jiffies or 8ms. With this timeout a timer
> list timer is queued. To make sure that timers will not fire early or
> race with a concurrent incrementation of jiffie, timers are queued
> always into the next bucket. As the timer will end up in the first level
> of the timer wheel the granularity of the buckets is 1 jiffies. This
> means that the timeout would be 3 jiffies in worst case.
>
> The additional jiffie in msleep() is the historical prevention that the
> sleep time is at least the specified time. This is handled by timer
> wheel core code itself, so this extra jiffie could be removed. I will
> provide a patch for it.

I missed to use the whole cc list above when sending the patch:

  https://lore.kernel.org/r/20240829074133.4547-1-anna-maria@linutronix.de/

Thanks,

	Anna-Maria