linux-kernel - Re: [PATCH rcu/next 2/3] rcu: Fix late wakeup when flush of bypass cblist happens (v6)

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Date:   Wed, 21 Sep 2022 07:54:15 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, rostedt@...dmis.org
Subject: Re: [PATCH rcu/next 2/3] rcu: Fix late wakeup when flush of bypass
 cblist happens (v6)

On Sat, Sep 17, 2022 at 04:41:59PM +0000, Joel Fernandes (Google) wrote:
> When the bypass cblist gets too big or its timeout has occurred, it is
> flushed into the main cblist. However, the bypass timer is still running
> and the behavior is that it would eventually expire and wake the GP
> thread.
> 
> Since we are going to use the bypass cblist for lazy CBs, do the wakeup
> soon as the flush for "too big or too long" bypass list happens.
> Otherwise, long delays can happen for callbacks which get promoted from
> lazy to non-lazy.
> 
> This is a good thing to do anyway (regardless of future lazy patches),
> since it makes the behavior consistent with behavior of other code paths
> where flushing into the ->cblist makes the GP kthread into a
> non-sleeping state quickly.
> 
> [ Frederic Weisbec: changes to not do wake up GP thread unless needed,
> 		    comment changes ].
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Queued and pushed this and 1/3, thank you both!

							Thanx, Paul

> ---
>  kernel/rcu/tree_nocb.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 0a5f0ef41484..04c87f250e01 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -433,8 +433,9 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>  	if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) ||
>  	    ncbs >= qhimark) {
>  		rcu_nocb_lock(rdp);
> +		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
> +
>  		if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
> -			*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
>  			if (*was_alldone)
>  				trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
>  						    TPS("FirstQ"));
> @@ -447,7 +448,12 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>  			rcu_advance_cbs_nowake(rdp->mynode, rdp);
>  			rdp->nocb_gp_adv_time = j;
>  		}
> -		rcu_nocb_unlock_irqrestore(rdp, flags);
> +
> +		// The flush succeeded and we moved CBs into the regular list.
> +		// Don't wait for the wake up timer as it may be too far ahead.
> +		// Wake up the GP thread now instead, if the cblist was empty.
> +		__call_rcu_nocb_wake(rdp, *was_alldone, flags);
> +
>  		return true; // Callback already enqueued.
>  	}
>  
> -- 
> 2.37.3.968.ga6b4b080e4-goog
>