linux-kernel - Re: [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass cblist happens

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Date:   Sat, 3 Sep 2022 07:04:03 -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,
        vineeth@...byteword.org, boqun.feng@...il.com
Subject: Re: [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass
 cblist happens

On Thu, Sep 01, 2022 at 10:17:06PM +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 happens. Otherwise, the lazy-timer will go off much
> later and the now-non-lazy cblist CBs can get stranded for the duration
> of the timer.
> 
> This is a good thing to do anyway (regardless of this series), since it
> makes the behavior consistent with behavior of other code paths where queueing
> something into the ->cblist makes the GP kthread in a non-sleeping state
> quickly.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree_nocb.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 0a5f0ef41484..31068dd31315 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -447,7 +447,13 @@ 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 ->cblist.
> +		// However, the bypass timer might still be running. Wakeup the
> +		// GP thread by calling a helper with was_all_done set so that
> +		// wake up happens (needed if main CB list was empty before).
> +		__call_rcu_nocb_wake(rdp, true, flags)

TREE01 and TREE04 gripe about the missing ";".  I added it.

							Thanx, Paul

> +
>  		return true; // Callback already enqueued.
>  	}
>  
> -- 
> 2.37.2.789.g6183377224-goog
>