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Date:   Tue, 6 Sep 2022 03:07:05 +0000
From:   Joel Fernandes <joel@joelfernandes.org>
To:     rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com,
        urezki@gmail.com, neeraj.iitr10@gmail.com, frederic@kernel.org,
        paulmck@kernel.org, rostedt@goodmis.org, vineeth@bitbyteword.org,
        boqun.feng@gmail.com
Subject: Re: [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass
 cblist happens
Message-ID: <Yxa5WVwwcDmCL9Dw@google.com>
References: <20220901221720.1105021-1-joel@joelfernandes.org>
 <20220901221720.1105021-5-joel@joelfernandes.org>
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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@joelfernandes.org>
> ---

Here is the updated version of this patch after review comments from
Frederic:

---8<-----------------------

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