linux-kernel - Re: [PATCH 5/6] posix-cpu-timers: Force next expiration recalc after early timer firing

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Message-ID: <20210616145309.GF801071@lothringen>
Date:   Wed, 16 Jun 2021 16:53:09 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 5/6] posix-cpu-timers: Force next expiration recalc after
 early timer firing

On Wed, Jun 16, 2021 at 03:23:50PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 16, 2021 at 01:59:23PM +0200, Frederic Weisbecker wrote:
> > On Wed, Jun 16, 2021 at 11:42:53AM +0200, Peter Zijlstra wrote:
> > > I'm thinking this is a better fix than patch #2. AFAICT you can now go
> > > back to unconditionally doing start, and then if we fire it early, we'll
> > > disarm the thing.
> > > 
> > > That would avoid the disconnect between the start condition and the fire
> > > condition.
> > 
> > Right but the drawback is that we unconditionally start the threadgroup
> > counter while initializing the timer to 0 (deactivated).
> > 
> > Then in the next tick at least one thread will need to lock the sighand
> > and re-evaluate the whole list.
> 
> Yes.. but how common is it to enqueue expired timers? Surely that's an
> unlikely corner case. All normal timers will have to suffer one extra
> tick and iteration on exit, so I find it hard to justify complexity to
> optimize an unlikely case.
> 
> I would rather have more obvious code.

Ok, I'm having a try at it.

Thanks!