Re: [PATCH v2 13/15] powerpc/rtas: Use fsleep() to minimize additional sleep duration

[Anna-Maria Behnsen <anna-maria@linutronix.de>]

Frederic Weisbecker <frederic@kernel.org> writes:

> Le Wed, Sep 11, 2024 at 07:13:39AM +0200, Anna-Maria Behnsen a écrit :
>> When commit 38f7b7067dae ("powerpc/rtas: rtas_busy_delay() improvements")
>> was introduced, documentation about proper usage of sleep realted functions
>
> related*
>
>> was outdated.
>> 
>> The commit message references the usage of a HZ=100 system. When using a
>> 20ms sleep duration on such a system and therefore using msleep(), the
>> possible additional slack will be +10ms.
>> 
>> When the system is configured with HZ=100 the granularity of a jiffy and of
>> a bucket of the lowest timer wheel level is 10ms. To make sure a timer will
>> not expire early (when queueing of the timer races with an concurrent
>> update of jiffies), timers are always queued into the next bucket. This is
>> the reason for the maximal possible slack of 10ms.
>> 
>> fsleep() limits the maximal possible slack to 25% by making threshold
>> between usleep_range() and msleep() HZ dependent. As soon as the accuracy
>> of msleep() is sufficient, the less expensive timer list timer based
>> sleeping function is used instead of the more expensive hrtimer based
>> usleep_range() function. The udelay() will not be used in this specific
>> usecase as the lowest sleep length is larger than 1 microsecond.
>
> Isn't udelay() for everything below 10us ?

It's larger than 1 millisecond...

>
>> 
>> Use fsleep() directly instead of using an own heuristic for the best
>> sleeping mechanism to use..
>> 
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Nathan Lynch <nathanl@linux.ibm.com>
>> Cc: linuxppc-dev@lists.ozlabs.org
>> Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
>> Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
>
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>