linux-kernel - Re: [PATCH] sched/fair: Make the BW replenish timer expire in hardirq context for PREEMPT

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Message-ID: <20231102161949.IdSqSK2E@linutronix.de>
Date:   Thu, 2 Nov 2023 17:19:49 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tomas Glozar <tglozar@...hat.com>
Subject: Re: [PATCH] sched/fair: Make the BW replenish timer expire in
 hardirq context for PREEMPT_RT

On 2023-10-31 17:01:20 [+0100], Peter Zijlstra wrote:
> On Mon, Oct 30, 2023 at 03:51:04PM +0100, Valentin Schneider wrote:
> > task would get PI'd to FIFO1 (ktimers' default priority). Unfortunately,
> > rwlocks cannot sanely do PI as they allow multiple readers.
…

> I'm thinking working on that is saner than adding this O(n) cgroup loop
> to hard-irq context. Hmm?

I have plans to get rid of the softirq issue and the argument for "bad"
or inefficient rwlocks is usually "get rid of rwlocks then". So…

Then I looked at the patch and it only swapped the flag nothing else and
this hardly works. So I looked at sched_cfs_period_timer():
| static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
| {
…
|         raw_spin_lock_irqsave(&cfs_b->lock, flags);
…
|         raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
| 
|         return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
|}

Judging by this, the whole callback runs already with disabled
interrupts. At least now it enabled interrupts if multiple callbacks are
invoked…

Sebastian