Re: [PATCH v3] sched/idle: disable tick in idle=poll idle entry

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Nov 05, 2025 at 09:57:48AM -0300, Marcelo Tosatti wrote:
> On Mon, Nov 03, 2025 at 01:30:23PM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 03, 2025 at 08:48:14AM -0300, Marcelo Tosatti wrote:
> > > 
> > > Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5
> > > ("tick/nohz: Conditionally restart tick on idle exit") allows
> > 
> > Quoting a commit usually shortens the hash to 12 charters, no?
> > 
> > > a nohz_full CPU to enter idle and return from it with the
> > > scheduler tick disabled (since the tick might be undesired noise).
> > > 
> > > The idle=poll case still unconditionally restarts the tick when entering
> > > idle.
> > > 
> > > To reduce the noise for that case as well, stop the tick when entering
> > > idle, for the idle=poll case.
> > > 
> > > Change tick_nohz_full_kick_cpu to set NEED_RESCHED bit, to handle the
> > > case where a new timer is added from an interrupt. This breaks out of
> > > cpu_idle_poll and rearms the timer if necessary.
> > > 
> > > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > > 
> > > ---
> > > 
> > > v3: Add comment with proper explanation (Frederic Weisbecker)
> > >     Add signed-off-by			(Thomas Gleixner)
> > > v2: Handle the case where a new timer is added from an interrupt (Frederic Weisbecker)
> > > 
> > >  include/linux/sched.h    |    2 ++
> > >  kernel/sched/core.c      |   10 ++++++++++
> > >  kernel/sched/idle.c      |    2 +-
> > >  kernel/time/tick-sched.c |    7 +++++++
> > >  4 files changed, 20 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index cbb7340c5866..1f6938dc20cd 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -2428,4 +2428,6 @@ extern void migrate_enable(void);
> > >  
> > >  DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
> > >  
> > > +void set_tif_resched_if_polling(int cpu);
> > > +
> > >  #endif
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index f1ebf67b48e2..f0b84600084b 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -988,6 +988,11 @@ static bool set_nr_if_polling(struct task_struct *p)
> > >  	return true;
> > >  }
> > >  
> > > +void set_tif_resched_if_polling(int cpu)
> > > +{
> > > +	set_nr_if_polling(cpu_rq(cpu)->idle);
> > > +}
> > > +
> > >  #else
> > >  static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
> > >  {
> > > @@ -999,6 +1004,11 @@ static inline bool set_nr_if_polling(struct task_struct *p)
> > >  {
> > >  	return false;
> > >  }
> > > +
> > > +void set_tif_resched_if_polling(int cpu)
> > > +{
> > > +	set_tsk_need_resched(cpu_rq(cpu)->idle);
> > > +}
> > >  #endif
> > >  
> > >  static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
> > > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > > index c39b089d4f09..428c2d1cbd1b 100644
> > > --- a/kernel/sched/idle.c
> > > +++ b/kernel/sched/idle.c
> > > @@ -324,7 +324,7 @@ static void do_idle(void)
> > >  		 * idle as we know that the IPI is going to arrive right away.
> > >  		 */
> > >  		if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
> > > -			tick_nohz_idle_restart_tick();
> > > +			tick_nohz_idle_stop_tick();
> > >  			cpu_idle_poll();
> > >  		} else {
> > >  			cpuidle_idle_call();
> > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > > index c527b421c865..9ec51da49591 100644
> > > --- a/kernel/time/tick-sched.c
> > > +++ b/kernel/time/tick-sched.c
> > > @@ -408,6 +408,13 @@ void tick_nohz_full_kick_cpu(int cpu)
> > >  	if (!tick_nohz_full_cpu(cpu))
> > >  		return;
> > >  
> > > +	/*
> > > +	 * When idle=poll, with the tick disabled (therefore idle CPU looping
> > > +	 * at cpu_idle_poll), if a new timer is added from an interrupt,
> > > +	 * the cpu_idle_poll only exits when TIF_NEED_RESCHED gets set.
> > > +	 */
> > > +	set_tif_resched_if_polling(cpu);
> > > +
> > >  	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
> > >  }
> > 
> > I'm confused. Why is this here and not in tick_nohz_start_idle() or
> > something?
> > 
> > 
> 
> Hi Peter,
> 
> The codepath being followed is:
> 
> enqueue_timer -> trigger_dyntick_cpu -> wake_up_nohz_cpu ->
> wake_up_full_nohz_cpu -> tick_nohz_full_kick_cpu ->
> set_tif_resched_if_polling. 
> 
> So we only set the PF_RESCHED bit if there is a pending timer
> on the CPU.
> 
> Calling unconditionally from tick_nohz_start_idle seems strange:
> 
> /**
>  * tick_nohz_idle_enter - prepare for entering idle on the current CPU
>  *
>  * Called when we start the idle loop.
>  */
> void tick_nohz_idle_enter(void)
> {
>         struct tick_sched *ts;
>         
>         lockdep_assert_irqs_enabled();
> 
>         local_irq_disable();
> 
>         ts = this_cpu_ptr(&tick_cpu_sched);
> 
>         WARN_ON_ONCE(ts->timer_expires_base);
> 
>         tick_sched_flag_set(ts, TS_FLAG_INIDLE);
>         tick_nohz_start_idle(ts);
> 
>         local_irq_enable();
> }
> 
> Can test for TS_FLAG_INIDLE before calling set_tif_resched_if_polling 
> (but seems not necessary since tick_nohz_full_kick_cpu will wake up the 
> CPU anyway and is a slow path (timer addition)).
> 
> What do you think?

OK, it looks like there are no further comments on this patch.

Frederic, Peter, Thomas, can you ACK ???

Thanks