linux-kernel - [PATCH rcu 09/18] rcu: Start timing stall repetitions after warning complete

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Date:   Wed, 21 Jul 2021 13:21:17 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [PATCH rcu 09/18] rcu: Start timing stall repetitions after warning complete

Systems with low-bandwidth consoles can have very large printk()
latencies, and on such systems it makes no sense to have the next RCU CPU
stall warning message start output before the prior message completed.
This commit therefore sets the time of the next stall only after the
prints have completed.  While printing, the time of the next stall
message is set to ULONG_MAX/2 jiffies into the future.

Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree_stall.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index e199022dce9dc..42847caa3909b 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -647,6 +647,7 @@ static void print_cpu_stall(unsigned long gps)
 
 static void check_cpu_stall(struct rcu_data *rdp)
 {
+	bool didstall = false;
 	unsigned long gs1;
 	unsigned long gs2;
 	unsigned long gps;
@@ -692,7 +693,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 	    ULONG_CMP_GE(gps, js))
 		return; /* No stall or GP completed since entering function. */
 	rnp = rdp->mynode;
-	jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
+	jn = jiffies + ULONG_MAX / 2;
 	if (rcu_gp_in_progress() &&
 	    (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
 	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
@@ -709,6 +710,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 		print_cpu_stall(gps);
 		if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
 			rcu_ftrace_dump(DUMP_ALL);
+		didstall = true;
 
 	} else if (rcu_gp_in_progress() &&
 		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
@@ -726,6 +728,11 @@ static void check_cpu_stall(struct rcu_data *rdp)
 		print_other_cpu_stall(gs2, gps);
 		if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
 			rcu_ftrace_dump(DUMP_ALL);
+		didstall = true;
+	}
+	if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
+		jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
+		WRITE_ONCE(rcu_state.jiffies_stall, jn);
 	}
 }
 
-- 
2.31.1.189.g2e36527f23