Re: [PATCH] tick/nohz: Narrow down noise while setting current task's tick dependency

[Frederic Weisbecker <frederic@kernel.org>]

On Mon, May 18, 2020 at 10:57:58AM +0200, Peter Zijlstra wrote:
> On Fri, May 15, 2020 at 02:34:29AM +0200, Frederic Weisbecker wrote:
> > So far setting a tick dependency on any task, including current, used to
> > trigger an IPI to all CPUs. That's of course suboptimal but it wasn't
> > an issue as long as it was only used by posix-cpu-timers on nohz_full,
> > a combo that nobody seemed to use in real life.
> > 
> > But RCU started to use task tick dependency on current task to fix
> > stall issues on callbacks processing. These trigger regular and
> > undesired system wide IPIs on nohz_full.
> > 
> > The fix is very easy while setting a tick dependency on the current
> > task, only its CPU needs an IPI.
> > 
> > Fixes: 6a949b7af82d (rcu: Force on tick when invoking lots of callbacks)
> > Reported-by: Matt Fleming <matt@codeblueprint.co.uk>
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: stable@kernel.org
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > ---
> >  kernel/time/tick-sched.c | 22 +++++++++++++++-------
> >  1 file changed, 15 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 3e2dc9b8858c..f0199a4ba1ad 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -351,16 +351,24 @@ void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
> >  EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);
> >  
> >  /*
> > - * Set a per-task tick dependency. Posix CPU timers need this in order to elapse
> > - * per task timers.
> > + * Set a per-task tick dependency. RCU need this. Also posix CPU timers
> > + * in order to elapse per task timers.
> >   */
> >  void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit)
> >  {
> > -	/*
> > -	 * We could optimize this with just kicking the target running the task
> > -	 * if that noise matters for nohz full users.
> > -	 */
> > -	tick_nohz_dep_set_all(&tsk->tick_dep_mask, bit);
> > +	if (!atomic_fetch_or(BIT(bit), &tsk->tick_dep_mask)) {
> 
> So why not simply:
> 
> 		tick_nohz_full_kick_cpu(task_cpu(tsk)); ?
> 
> If it got preempted, the scheduling involved would already have observed
> the bit we just set and kept the tick on anyway, same for migration.
> 
> Or am I missing something?

Hmm, I guess we would then need some sort of ordering like
this:

         CPU 0                            CPU 1
         -----                            -----
         p->cpu = smp_processor_id()      atomic_fetch_or(bit, p->tick_dep_mask)
         smp_mb();                        smp_mb(); //actually implied by atomic_fetch_or()
         READ p->tick_dep_mask            irq_work_on(...., p->cpu)

And since p->cpu is already set and visible on context switch, it should work
indeed. Now in the case CPU 1 reads a stale task_cpu(), that's fine as CPU 0 sees
the new tick_dep_mask, but CPU 1 might be dealing with an offlined CPU, right?

So I guess I should still protect against hotplug with cpus_read_lock() but
tick_nohz_dep_set_task() isn't supposed to sleep...

Or preempt_disable() could help us with that somehow? I'm always confused with
the guarantees that disabled preemption can offer toward hotplug.


> 
> > +		if (tsk == current) {
> > +			preempt_disable();
> > +			tick_nohz_full_kick();
> > +			preempt_enable();
> > +		} else {
> > +			/*
> > +			 * Some future tick_nohz_full_kick_task()
> > +			 * should optimize this.
> > +			 */
> > +			tick_nohz_full_kick_all();
> > +		}
> > +	}
>