linux-kernel - [PATCH 3/4] stop_machine: Rearrange multi_cpu

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Date: Tue, 25 Jun 2024 21:42:46 +1000
From: Nicholas Piggin <npiggin@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: Nicholas Piggin <npiggin@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/4] stop_machine: Rearrange multi_cpu_stop state machine loop

More clearly separate the state-machine progress case from the
non-progress case.

Move stop_machine_yield() and rcu_momentary_dyntick_idle() calls
to the non-progress case like touch_nmi_watchdog(), rather than
always calling them, since there is no reason to yield or touch
watchdogs if the state machine progressed.

Signed-off-by: Nicholas Piggin <npiggin@...il.com>
---
 kernel/stop_machine.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index cedb17ba158a..1e5c4702e36c 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -225,8 +225,6 @@ static int multi_cpu_stop(void *data)
 
 	/* Simple state machine */
 	do {
-		/* Chill out and ensure we re-read multi_stop_state. */
-		stop_machine_yield(cpumask);
 		newstate = READ_ONCE(msdata->state);
 		if (newstate != curstate) {
 			curstate = newstate;
@@ -243,15 +241,22 @@ static int multi_cpu_stop(void *data)
 				break;
 			}
 			ack_state(msdata);
-		} else if (curstate > MULTI_STOP_PREPARE) {
-			/*
-			 * At this stage all other CPUs we depend on must spin
-			 * in the same loop. Any reason for hard-lockup should
-			 * be detected and reported on their side.
-			 */
-			touch_nmi_watchdog();
+
+		} else {
+			/* No state change, chill out */
+			stop_machine_yield(cpumask);
+			if (curstate > MULTI_STOP_PREPARE) {
+				/*
+				 * At this stage all other CPUs we depend on
+				 * must spin in the same loop. Any reason for
+				 * hard-lockup should be detected and reported
+				 * on their side.
+				 */
+				touch_nmi_watchdog();
+			}
+			rcu_momentary_dyntick_idle();
 		}
-		rcu_momentary_dyntick_idle();
+
 	} while (curstate != MULTI_STOP_EXIT);
 
 	local_irq_restore(flags);
-- 
2.45.1