linux-kernel - [tip: core/rcu] rcu/tree: Force quiescent state on callback overload

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Message-ID: <160222534130.7002.18129565337789660727.tip-bot2@tip-bot2>
Date:   Fri, 09 Oct 2020 06:35:41 -0000
From:   "tip-bot2 for Neeraj Upadhyay" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: core/rcu] rcu/tree: Force quiescent state on callback overload

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     9c39245382de4d52a122641952900709d4a9950b
Gitweb:        https://git.kernel.org/tip/9c39245382de4d52a122641952900709d4a9950b
Author:        Neeraj Upadhyay <neeraju@...eaurora.org>
AuthorDate:    Mon, 22 Jun 2020 00:07:27 +05:30
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 24 Aug 2020 18:36:05 -07:00

rcu/tree: Force quiescent state on callback overload

On callback overload, it is necessary to quickly detect idle CPUs,
and rcu_gp_fqs_check_wake() checks for this condition.  Unfortunately,
the code following the call to this function does not repeat this check,
which means that in reality no actual quiescent-state forcing, instead
only a couple of quick and pointless wakeups at the beginning of the
grace period.

This commit therefore adds a check for the RCU_GP_FLAG_OVLD flag in
the post-wakeup "if" statement in rcu_gp_fqs_loop().

Fixes: 1fca4d12f4637 ("rcu: Expedite first two FQS scans under callback-overload conditions")
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8969120..4770d77 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1884,7 +1884,7 @@ static void rcu_gp_fqs_loop(void)
 			break;
 		/* If time for quiescent-state forcing, do it. */
 		if (!time_after(rcu_state.jiffies_force_qs, jiffies) ||
-		    (gf & RCU_GP_FLAG_FQS)) {
+		    (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) {
 			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
 					       TPS("fqsstart"));
 			rcu_gp_fqs(first_gp_fqs);