From: Marcelo Tosatti <mtosatti@redhat.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org,
Nitesh Narayan Lal <nitesh@redhat.com>,
Peter Xu <peterx@redhat.com>
Subject: Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task
Date: Tue, 13 Oct 2020 14:13:28 -0300 [thread overview]
Message-ID: <20201013171328.GA19284@fuller.cnet> (raw)
In-Reply-To: <20201008195444.GB86389@lothringen>
On Thu, Oct 08, 2020 at 09:54:44PM +0200, Frederic Weisbecker wrote:
> On Thu, Oct 08, 2020 at 02:54:09PM -0300, Marcelo Tosatti wrote:
> > On Thu, Oct 08, 2020 at 02:22:56PM +0200, Peter Zijlstra wrote:
> > > On Wed, Oct 07, 2020 at 03:01:52PM -0300, Marcelo Tosatti wrote:
> > > > When adding a tick dependency to a task, its necessary to
> > > > wakeup the CPU where the task resides to reevaluate tick
> > > > dependencies on that CPU.
> > > >
> > > > However the current code wakes up all nohz_full CPUs, which
> > > > is unnecessary.
> > > >
> > > > Switch to waking up a single CPU, by using ordering of writes
> > > > to task->cpu and task->tick_dep_mask.
> > > >
> > > > From: Frederic Weisbecker <frederic@kernel.org>
> > > > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > > > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > > >
> > > > Index: linux-2.6/kernel/time/tick-sched.c
> > > > ===================================================================
> > > > --- linux-2.6.orig/kernel/time/tick-sched.c
> > > > +++ linux-2.6/kernel/time/tick-sched.c
> > > > @@ -274,6 +274,31 @@ void tick_nohz_full_kick_cpu(int cpu)
> > > > irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
> > > > }
> > > >
> > > > +static void tick_nohz_kick_task(struct task_struct *tsk)
> > > > +{
> > > > + int cpu = task_cpu(tsk);
> > > > +
> > > > + /*
> > > > + * If the task concurrently migrates to another cpu,
> > > > + * we guarantee it sees the new tick dependency upon
> > > > + * schedule.
> > > > + *
> > > > + *
> > > > + * set_task_cpu(p, cpu);
> > > > + * STORE p->cpu = @cpu
> > > > + * __schedule() (switch to task 'p')
> > > > + * LOCK rq->lock
> > > > + * smp_mb__after_spin_lock() STORE p->tick_dep_mask
> > > > + * tick_nohz_task_switch() smp_mb() (atomic_fetch_or())
> > > > + * LOAD p->tick_dep_mask LOAD p->cpu
> > > > + */
> > > > +
> > > > + preempt_disable();
> > > > + if (cpu_online(cpu))
> > > > + tick_nohz_full_kick_cpu(cpu);
> > > > + preempt_enable();
> > > > +}
> > >
> > > So we need to kick the CPU unconditionally, or only when the task is
> > > actually running? AFAICT we only care about current->tick_dep_mask.
> >
> > tick is necessary to execute run_posix_cpu_timers, from tick interrupt,
> > even if task is not running.
>
> Yes but if the task isn't running, run_posix_cpu_timers() doesn't have
> anything to elapse. So indeed we can spare the IPI if the task is not
> running. Provided ordering makes sure that the task sees the new dependency
> when it schedules in of course.
True.
* p->on_cpu <- { 0, 1 }:
*
* is set by prepare_task() and cleared by finish_task() such that it will be
* set before p is scheduled-in and cleared after p is scheduled-out, both
* under rq->lock. Non-zero indicates the task is running on its CPU.
CPU-0 (tick_set_dep) CPU-1 (task switch)
STORE p->tick_dep_mask
smp_mb() (atomic_fetch_or())
LOAD p->on_cpu
context_switch(prev, next)
STORE next->on_cpu = 1
... [*]
LOAD current->tick_dep_mask
Don't see any explicit memory barrier in the [*] section?
next prev parent reply other threads:[~2020-10-13 17:14 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-07 18:01 [patch 0/2] nohz_full: only wakeup target CPUs when notifying new tick dependency (v2) Marcelo Tosatti
2020-10-07 18:01 ` [patch 1/2] nohz: only wakeup a single target cpu when kicking a task Marcelo Tosatti
2020-10-08 12:22 ` Peter Zijlstra
2020-10-08 17:54 ` Marcelo Tosatti
2020-10-08 19:54 ` Frederic Weisbecker
2020-10-13 17:13 ` Marcelo Tosatti [this message]
2020-10-14 8:33 ` Peter Zijlstra
2020-10-14 23:40 ` Frederic Weisbecker
2020-10-15 10:12 ` Peter Zijlstra
2020-10-26 14:42 ` Frederic Weisbecker
2020-10-20 18:52 ` Marcelo Tosatti
2020-10-22 12:53 ` Frederic Weisbecker
2020-10-08 14:59 ` Peter Xu
2020-10-08 15:28 ` Peter Zijlstra
2020-10-08 19:16 ` Peter Xu
2020-10-08 19:48 ` Frederic Weisbecker
2020-10-08 17:43 ` Marcelo Tosatti
2020-10-07 18:01 ` [patch 2/2] nohz: change signal tick dependency to wakeup CPUs of member tasks Marcelo Tosatti
2020-10-08 12:35 ` Peter Zijlstra
2020-10-08 18:04 ` Marcelo Tosatti
2020-10-08 19:11 [patch 0/2] nohz_full: only wakeup target CPUs when notifying new tick dependency (v3) Marcelo Tosatti
2020-10-08 19:11 ` [patch 1/2] nohz: only wakeup a single target cpu when kicking a task Marcelo Tosatti
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