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From: Dexuan Cui <decui@microsoft.com>
To: Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Dmitry Vyukov <dvyukov@google.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	KY Srinivasan <kys@microsoft.com>,
	Jork Loeser <Jork.Loeser@microsoft.com>,
	Rakib Mullick <rakib.mullick@gmail.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Josh Poulson <jopoulso@microsoft.com>,
	"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>
Subject: [PATCH] tick/broadcast: Use for_each_cpu() specially on UP kernels
Date: Tue, 15 May 2018 19:52:50 +0000	[thread overview]
Message-ID: <KL1P15301MB0006FA63BC22BEB64902EAA0BF930@KL1P15301MB0006.APCP153.PROD.OUTLOOK.COM> (raw)


for_each_cpu() unintuitively reports CPU0 as set independent of the actual
cpumask content on UP kernels. This causes an unexpected PIT interrupt
storm on a UP kernel running in an SMP virtual machine on Hyper-V, and as
a result, the virtual machine can suffer from a strange random delay of 1~20
minutes during boot-up, and sometimes it can hang forever.

Link: https://lkml.org/lkml/2018/5/9/63
Link: https://lkml.org/lkml/2018/5/15/747
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: stable@vger.kernel.org
---

Some part of the changelog and comments are copied from 
Thomas Gleixner's 115ef3b7e61a ("watchdog/hardlockup/perf: Cure UP damage") :-)

 kernel/time/tick-broadcast.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index b398c2e..2fc59ad 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -612,6 +612,14 @@ static void tick_handle_oneshot_broadcast(struct clock_event_device *dev)
 	now = ktime_get();
 	/* Find all expired events */
 	for_each_cpu(cpu, tick_broadcast_oneshot_mask) {
+#ifndef CONFIG_SMP
+		/*
+		 * Required because for_each_cpu() reports unconditionally
+		 * CPU0 as set on UP kernels.
+		 */
+		if (cpumask_empty(tick_broadcast_oneshot_mask))
+			break;
+#endif
 		td = &per_cpu(tick_cpu_device, cpu);
 		if (td->evtdev->next_event <= now) {
 			cpumask_set_cpu(cpu, tmpmask);
-- 
2.7.4

         reply	other threads:[~2018-05-15 19:52 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-09  6:24 for_each_cpu() is buggy for UP kernel? Dexuan Cui
2018-05-09 23:20 ` Andrew Morton
2018-05-13 13:35   ` Thomas Gleixner
2018-05-13 18:21 ` Linus Torvalds
2018-05-14  7:28   ` Dmitry Vyukov
2018-05-15  3:02   ` Dexuan Cui
2018-05-15 17:21     ` Linus Torvalds
2018-05-15 20:10       ` Dexuan Cui
2018-05-15 19:52 ` Dexuan Cui [this message]
2018-05-15 20:48   ` [tip:timers/urgent] tick/broadcast: Use for_each_cpu() specially on UP kernels tip-bot for Dexuan Cui

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