On Mon, Jan 09, 2023 at 06:20:55PM -0500, Joel Fernandes wrote:
>
>
> > On Jan 9, 2023, at 6:14 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Mon, Jan 09, 2023 at 10:17:56PM +0000, Joel Fernandes (Google) wrote:
> >> During suspend, we see failures to suspend 1 in 300-500 suspends.
> >> Looking closer, it appears that we are queuing lazy callbacks even
> >> though rcu_gp_is_expedited(). These delays appear to not be very welcome
> >> by the suspend/resume code as evidenced by these occasional suspend
> >> failures.
> >>
> >> This commit therefore checks if rcu_gp_is_expedited() and ignores the
> >> lazy hint if so.
> >>
> >> Ignoring the lazy hint if rcu_gp_is_expedited() makes the 3000
> >> suspend/resume cycles pass reliably on a 12th gen 12-core Intel CPU.
> >
> > Yow!!! ;-)
>
> :-D
>
> >> Fixes: 3cb278e73be5 ("rcu: Make call_rcu() lazy to save power")
> >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> >> ---
> >> Paul, could we take this for 6.2 -rc cycle? Thanks.
> >>
> >> kernel/rcu/tree.c | 5 +++--
> >> 1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >> index 63545d79da51..93eb03f8ed99 100644
> >> --- a/kernel/rcu/tree.c
> >> +++ b/kernel/rcu/tree.c
> >> @@ -2594,12 +2594,12 @@ static void check_cb_ovld(struct rcu_data *rdp)
> >> }
> >>
> >> static void
> >> -__call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy)
> >> +__call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
> >> {
> >> static atomic_t doublefrees;
> >> unsigned long flags;
> >> struct rcu_data *rdp;
> >> - bool was_alldone;
> >> + bool was_alldone, lazy;
> >
> > Please put "lazy" in alpha order. Except that...
>
> Ah sure.
>
> >
> >> /* Misaligned rcu_head! */
> >> WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
> >> @@ -2622,6 +2622,7 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy)
> >> kasan_record_aux_stack_noalloc(head);
> >> local_irq_save(flags);
> >> rdp = this_cpu_ptr(&rcu_data);
> >> + lazy = lazy_in && !rcu_gp_is_expedited();
> >
> > Doesn't this completely disable laziness on Android?
>
> Good point, I am not sure but it could be. Maybe it is safer that I add
> a new suspend-indicator then, with corresponding
> suspend entry/exit calls like we do for expedited.
>
> That way anyone doing it this way will not disable
> lazy fully.
>
> Thoughts?
Makes sense to me!
Just so you know, there is an overlapping patch series in flight here:
https://lore.kernel.org/all/20221219202910.3063036-1-elliott@xxxxxxx/
Thanx, Paul
