linux-kernel - Re: [PATCH -rcu] rcu: Disable lazy if call

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Message-ID: <20230109231416.GA4028633@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 9 Jan 2023 15:14:16 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        foo@....com
Subject: Re: [PATCH -rcu] rcu: Disable lazy if call_rcu() called when GPs
 expedited

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!!!  ;-)

> Fixes: 3cb278e73be5 ("rcu: Make call_rcu() lazy to save power")
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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...

>  	/* 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?

							Thanx, Paul

>  	/* Add the callback to our list. */
>  	if (unlikely(!rcu_segcblist_is_enabled(&rdp->cblist))) {
> -- 
> 2.39.0.314.g84b9a713c41-goog