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Date:   Mon, 9 Jan 2023 15:55:31 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        foo@bar.com
Subject: Re: [PATCH -rcu] rcu: Disable lazy if call_rcu() called when GPs
 expedited
Message-ID: <20230109235531.GB4028633@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20230109231416.GA4028633@paulmck-ThinkPad-P17-Gen-1>
 <C461EF52-B060-4871-8C20-30824983E787@joelfernandes.org>
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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@kernel.org> 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@joelfernandes.org>
> >> ---
> >> 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@hpe.com/

							Thanx, Paul