linux-kernel - Re: [PATCH v2 0/1] hrtimer: More fixes for handling of timer slack of rt tasks

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Message-ID: <46ad86f8b1fc4fb5dbb3da8f6ab664f48955849f.camel@siemens.com>
Date: Fri, 9 Aug 2024 08:47:29 +0000
From: "MOESSBAUER, Felix" <felix.moessbauer@...mens.com>
To: "qyousef@...alina.io" <qyousef@...alina.io>
CC: "tglx@...utronix.de" <tglx@...utronix.de>, "frederic@...nel.org"
	<frederic@...nel.org>, "Kiszka, Jan" <jan.kiszka@...mens.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"bigeasy@...utronix.de" <bigeasy@...utronix.de>, "anna-maria@...utronix.de"
	<anna-maria@...utronix.de>
Subject: Re: [PATCH v2 0/1] hrtimer: More fixes for handling of timer slack of
 rt tasks

On Fri, 2024-08-09 at 02:34 +0100, Qais Yousef wrote:
> On 08/05/24 16:09, Felix Moessbauer wrote:
> > This series fixes the (hopefully) last location of an incorrectly
> > handled timer slack on rt tasks in hrtimer_start_range_ns(), which
> > was
> > uncovered by a userland change in glibc 2.33.
> > 
> > Changes since v1:
> > 
> > - drop patch "hrtimer: Document, that PI boosted tasks have no
> > timer slack", as
> >   this behavior is incorrect and is already adressed in
> > 20240610192018.1567075-1-qyousef@...alina.io
> 
> There was discussion about this hrtimer usage in earlier version if
> it helps to
> come up with a potentially better patch

Hi, Sebastian already pointed me to this thread.

When debugging my issue, I did not know about it but was scratching my
head if the behavior / usage of rt_task is actually correct.
The whole naming was quite confusing. Many thanks for cleaning that up.

> 
>         
> https://lore.kernel.org/lkml/20240521110035.KRIwllGe@linutronix.de/
> 
> My patches got picked up by the way, you'd probably want to rebase
> and resend
> as now the function is named rt_or_dl_task_policy()

As we use rt_or_dl_task() in nanosleep, I'm wondering if we should use
the same in hrtimer_start_range_ns(). Is that because PI boosted tasks
need to acquire a lock which can only be a mutex_t or equivalent
sleeping lock on PREEMPT_RT?

Anyways, I'm thinking about getting rid of the policy based delta=0 and
just set the task->timer_slack_ns to 0 when changing the scheduling
policy (and changing it back to the default when reverting to
SCHED_OTHER). By that, we can get rid of the special handling and users
of the procfs would also see correct data in /timerslack_ns.

Felix

> 
> 
> Cheers
> 
> --
> Qais Yousef
> 
> > - use task_is_realtime() instead of rt_task()
> > - fix style of commit message
> > 
> > v1 discussion:
> > https://lore.kernel.org/lkml/20240805124116.21394-1-felix.moessbauer@siemens.com
> > 
> > Best regards,
> > Felix Moessbauer
> > Siemens AG
> > 
> > Felix Moessbauer (1):
> >   hrtimer: Ignore slack time for RT tasks in
> > hrtimer_start_range_ns()
> > 
> >  kernel/time/hrtimer.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > -- 
> > 2.39.2
> > 

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