From: Frederic Weisbecker <frederic@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
"Eric W . Biederman" <ebiederm@xmission.com>,
Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 5/6] posix-cpu-timers: Force next expiration recalc after early timer firing
Date: Wed, 16 Jun 2021 13:59:23 +0200 [thread overview]
Message-ID: <20210616115923.GC801071@lothringen> (raw)
In-Reply-To: <YMnHnUcufPhtnDZP@hirez.programming.kicks-ass.net>
On Wed, Jun 16, 2021 at 11:42:53AM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 01:31:58PM +0200, Frederic Weisbecker wrote:
> > diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
> > index 0b5715c8db04..d8325a906314 100644
> > --- a/kernel/time/posix-cpu-timers.c
> > +++ b/kernel/time/posix-cpu-timers.c
> > @@ -405,6 +405,21 @@ static int posix_cpu_timer_create(struct k_itimer *new_timer)
> > return 0;
> > }
> >
> > +static void __disarm_timer(struct k_itimer *timer, struct task_struct *p,
> > + u64 old_expires)
> > +{
> > + int clkidx = CPUCLOCK_WHICH(timer->it_clock);
> > + struct posix_cputimer_base *base;
> > +
> > + if (CPUCLOCK_PERTHREAD(timer->it_clock))
> > + base = p->posix_cputimers.bases + clkidx;
> > + else
> > + base = p->signal->posix_cputimers.bases + clkidx;
> > +
> > + if (old_expires == base->nextevt)
> > + base->nextevt = 0;
> > +}
> > +
> > /*
> > * Dequeue the timer and reset the base if it was its earliest expiration.
> > * It makes sure the next tick recalculates the base next expiration so we
> > @@ -415,24 +430,14 @@ static void disarm_timer(struct k_itimer *timer, struct task_struct *p)
> > {
> > struct cpu_timer *ctmr = &timer->it.cpu;
> > u64 old_expires = cpu_timer_getexpires(ctmr);
> > - struct posix_cputimer_base *base;
> > bool queued;
> > - int clkidx;
> >
> > queued = cpu_timer_dequeue(ctmr);
> > cpu_timer_setexpires(ctmr, 0);
> > if (!queued)
> > return;
> >
> > - clkidx = CPUCLOCK_WHICH(timer->it_clock);
> > -
> > - if (CPUCLOCK_PERTHREAD(timer->it_clock))
> > - base = p->posix_cputimers.bases + clkidx;
> > - else
> > - base = p->signal->posix_cputimers.bases + clkidx;
> > -
> > - if (old_expires == base->nextevt)
> > - base->nextevt = 0;
> > + __disarm_timer(timer, p, old_expires);
> > }
> >
> >
> > @@ -686,8 +691,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
> > u64 exp = bump_cpu_timer(timer, val);
> >
> > if (val < exp) {
> > - old_expires = exp - val;
> > - old->it_value = ns_to_timespec64(old_expires);
> > + old->it_value = ns_to_timespec64(exp - val);
> > } else {
> > old->it_value.tv_nsec = 1;
> > old->it_value.tv_sec = 0;
> > @@ -748,9 +752,28 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
> > * accumulate more time on this clock.
> > */
> > cpu_timer_fire(timer);
> > +
> > + sighand = lock_task_sighand(p, &flags);
> > + if (sighand == NULL)
> > + goto out;
> > + if (!cpu_timer_queued(&timer->it.cpu)) {
> > + /*
> > + * Disarm the previous timer to deactivate the tick
> > + * dependency and process wide cputime counter if
> > + * necessary.
> > + */
> > + __disarm_timer(timer, p, old_expires);
> > + /*
> > + * If the previous timer was deactivated, we might have
> > + * just started the process wide cputime counter. Make
> > + * sure we poke the tick to deactivate it then.
> > + */
> > + if (!old_expires && !CPUCLOCK_PERTHREAD(timer->it_clock))
> > + p->signal->posix_cputimers.bases[clkid].nextevt = 0;
> > + }
> > + unlock_task_sighand(p, &flags);
> > }
>
> 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.
next prev parent reply other threads:[~2021-06-16 11:59 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-04 11:31 [PATCH 0/6] posix-cpu-timers: Bunch of fixes Frederic Weisbecker
2021-06-04 11:31 ` [PATCH 1/6] posix-cpu-timers: Fix rearm racing against process tick Frederic Weisbecker
2021-06-09 11:54 ` Frederic Weisbecker
2021-06-11 11:49 ` Peter Zijlstra
2021-06-11 12:37 ` Frederic Weisbecker
2021-06-04 11:31 ` [PATCH 2/6] posix-cpu-timers: Don't start process wide cputime counter if timer is disabled Frederic Weisbecker
2021-06-09 12:18 ` Frederic Weisbecker
2021-06-10 10:24 ` Frederic Weisbecker
2021-06-16 8:51 ` Peter Zijlstra
2021-06-16 10:51 ` Frederic Weisbecker
2021-06-16 11:26 ` Peter Zijlstra
2021-06-16 11:50 ` Peter Zijlstra
2021-06-04 11:31 ` [PATCH 3/6] posix-cpu-timers: Force next_expiration recalc after timer deletion Frederic Weisbecker
2021-06-16 9:16 ` Peter Zijlstra
2021-06-04 11:31 ` [PATCH 4/6] posix-cpu-timers: Force next_expiration recalc after timer reset Frederic Weisbecker
2021-06-16 9:23 ` Peter Zijlstra
2021-06-16 11:21 ` Frederic Weisbecker
2021-06-16 11:33 ` Peter Zijlstra
2021-06-04 11:31 ` [PATCH 5/6] posix-cpu-timers: Force next expiration recalc after early timer firing Frederic Weisbecker
2021-06-16 9:42 ` Peter Zijlstra
2021-06-16 11:59 ` Frederic Weisbecker [this message]
2021-06-16 13:23 ` Peter Zijlstra
2021-06-16 14:53 ` Frederic Weisbecker
2021-06-04 11:31 ` [PATCH 6/6] posix-cpu-timers: Force next expiration recalc after itimer reset Frederic Weisbecker
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