linux-kernel - Re: [PATCH] rcu: Reduce synchronize_rcu() delays when all wait heads are in use

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Message-ID: <ZfHcvI54IrbQTjbP@localhost.localdomain>
Date: Wed, 13 Mar 2024 18:05:00 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
Cc: paulmck@...nel.org, joel@...lfernandes.org, josh@...htriplett.org,
	boqun.feng@...il.com, rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
	qiang.zhang1211@...il.com, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, neeraj.upadhyay@...nel.org
Subject: Re: [PATCH] rcu: Reduce synchronize_rcu() delays when all wait heads
 are in use

Le Wed, Mar 13, 2024 at 09:41:58PM +0530, Neeraj Upadhyay a écrit :
> > Also there is a risk that this non-wait-head gets later assigned as
> > rcu_state.srs_done_tail. And then this pending sr may not be completed
> > until the next grace period calling rcu_sr_normal_gp_cleanup()? (Because
> > the work doesn't take care of rcu_state.srs_done_tail itself). And then
> > the delay can be arbitrary.
> > 
> 
> That is correct. Only the first node suffers from deferred GP.
> If there are large number of callbacks which got added after
> last available wait head was queued, all those callbacks (except one)
> can still have a GP assigned to them.

Oh and yes I missed the fact that you still reissue a GP while queueing
a non wait-head as ->srs_wait_tail. That point looks good.

Thanks.