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

[Frederic Weisbecker <frederic@kernel.org>]

On Tue, Sep 06, 2022 at 03:07:05AM +0000, Joel Fernandes wrote:
> On Thu, Sep 01, 2022 at 10:17:06PM +0000, Joel Fernandes (Google) wrote:
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> Subject: [PATCH v6] rcu: Fix late wakeup when flush of bypass cblist happens
> 
> 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.
> 
> [ Frederic Weisbec: changes to not do wake up GP thread unless needed ].
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/tree_nocb.h | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 0a5f0ef41484..4dc86274b3e8 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 +448,13 @@ 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

That part of the comment mentioning the bypass timer looks strange...


> +		// GP thread by calling a helper with was_all_done set so that
> +		// wake up happens (needed if main CB list was empty before).

How about:

                // 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 now GP thread instead if the cblist was empty

Thanks.

Other than that the patch looks good, thanks!

> +		__call_rcu_nocb_wake(rdp, *was_alldone, flags);
> +
>  		return true; // Callback already enqueued.
>  	}
>  
> -- 
> 2.37.2.789.g6183377224-goog
>