linux-kernel - [PATCH v2] kernel/sysrq: reset watchdog on all CPUs while processing sysrq-w

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Date:	Thu, 9 Jun 2016 15:20:05 +0300
From:	Andrey Ryabinin <aryabinin@...tuozzo.com>
To:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: [PATCH v2] kernel/sysrq: reset watchdog on all CPUs while processing sysrq-w

Lengthy output of sysrq-w may take a lot of time on slow serial console.
Currently we reset NMI-watchdog on the current CPU to avoid spurious
lockup messages. Sometimes this doesn't work since softlockup watchdog
might trigger on another CPU which is waiting for an IPI to proceed.
We reset softlockup watchdogs on all CPUs, but we do this only after
listing all tasks, and this may be too late on a busy system.

So, reset watchdogs CPUs earlier, in for_each_process_thread() loop.

Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---

Changes since v1:
 - keep touch_nmi_watchdog()
 - touch_all_softlockup_watchdogs() after the loop becomes redundant, so remove it.
 - added comment.

 kernel/sched/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7f2cae4..32fe0a7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5133,14 +5133,16 @@ void show_state_filter(unsigned long state_filter)
 		/*
 		 * reset the NMI-timeout, listing all files on a slow
 		 * console might take a lot of time:
+		 * Also, reset softlockup watchdogs on all CPUs, because
+		 * another CPU might be blocked waiting for us to process
+		 * an IPI.
 		 */
 		touch_nmi_watchdog();
+		touch_all_softlockup_watchdogs();
 		if (!state_filter || (p->state & state_filter))
 			sched_show_task(p);
 	}
 
-	touch_all_softlockup_watchdogs();
-
 #ifdef CONFIG_SCHED_DEBUG
 	if (!state_filter)
 		sysrq_sched_debug_show();
-- 
2.7.3