Re: [PATCH 4/9] timers: Always keep track of next expiry

Frederic Weisbecker
2020-07-17T12:51:47Z

On Tue, Jul 14, 2020 at 10:49:28AM +0200, Anna-Maria Behnsen wrote:
> Hi Frederic,
>
> On Tue, 7 Jul 2020, Frederic Weisbecker wrote:
>
> > So far next expiry was only tracked while the CPU was in nohz_idle mode
> > in order to cope with missing ticks that can't increment the base->clk
> > periodically anymore.
> >
> > We are going to expand that logic beyond nohz in order to spare timers
> > softirqs so do it unconditionally.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
> > Cc: Juri Lelli <juri.lelli@redhat.com>
> > ---
> > kernel/time/timer.c | 36 +++++++++++++++++++-----------------
> > 1 file changed, 19 insertions(+), 17 deletions(-)
> >
> > diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> > index acf7cb8c09f8..8a4138e47aa4 100644
> > --- a/kernel/time/timer.c
> > +++ b/kernel/time/timer.c
> > @@ -558,8 +558,22 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
> > * timer is not deferrable. If the other CPU is on the way to idle
> > * then it can't set base->is_idle as we hold the base lock:
> > */
> > - if (!base->is_idle)
> > - return;
> > + if (base->is_idle)
> > + wake_up_nohz_cpu(base->cpu);
> > +}
> > +
> > +/*
> > + * Enqueue the timer into the hash bucket, mark it pending in
> > + * the bitmap and store the index in the timer flags.
> > + */
> > +static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
> > + unsigned int idx)
> > +{
> > + hlist_add_head(&timer->entry, base->vectors + idx);
> > + __set_bit(idx, base->pending_map);
> > + timer_set_idx(timer, idx);
> > +
> > + trace_timer_start(timer, timer->expires, timer->flags);
> >
> > /* Check whether this is the new first expiring timer: */
> > if (time_after_eq(timer->expires, base->next_expiry))
> > @@ -578,21 +592,7 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
> > } else {
> > base->next_expiry = timer->expires;
> > }
> > - wake_up_nohz_cpu(base->cpu);
> > -}
> >
> > -/*
> > - * Enqueue the timer into the hash bucket, mark it pending in
> > - * the bitmap and store the index in the timer flags.
> > - */
> > -static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
> > - unsigned int idx)
> > -{
> > - hlist_add_head(&timer->entry, base->vectors + idx);
> > - __set_bit(idx, base->pending_map);
> > - timer_set_idx(timer, idx);
> > -
> > - trace_timer_start(timer, timer->expires, timer->flags);
> > trigger_dyntick_cpu(base, timer);
> > }
> >
>
> Could you please split those two hunks which do only a restructuring into a
> separate patch?

The problem is that those hunks are not only a restructuring but they also
change the way we update next_expiry, since we do it outside idle context.
And that update won't make sense without the proper initialization of next_expiry
that comes later in the patch.

Thanks.