linux-kernel - Re: [PATCH v2 5/8] rcu/nocb: Wake up gp thread when flushing

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Message-ID: <20220626040622.GM1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Sat, 25 Jun 2022 21:06:22 -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
Subject: Re: [PATCH v2 5/8] rcu/nocb: Wake up gp thread when flushing

On Wed, Jun 22, 2022 at 10:50:59PM +0000, Joel Fernandes (Google) wrote:
> We notice that rcu_barrier() can take a really long time. It appears
> that this can happen when all CBs are lazy and the timer does not fire
> yet. So after flushing, nothing wakes up GP thread. This patch forces
> GP thread to wake when bypass flushing happens, this fixes the
> rcu_barrier() delays with lazy CBs.

I am wondering if there is a bug in non-rcu_barrier() lazy callback
processing hiding here as well?

							Thanx, Paul

> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree_nocb.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 2f5da12811a5..b481f1ea57c0 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -325,6 +325,8 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>  	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
>  	WRITE_ONCE(rdp->nocb_bypass_first, j);
>  	rcu_nocb_bypass_unlock(rdp);
> +
> +	wake_nocb_gp(rdp, true);
>  	return true;
>  }
>  
> -- 
> 2.37.0.rc0.104.g0611611a94-goog
>