linux-kernel - [PATCH 3.2 003/152] sched/cputime: Fix prev steal time accouting during CPU hotplug

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Message-ID: <lsq.1479082447.722410790@decadent.org.uk>
Date:   Mon, 14 Nov 2016 00:14:07 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Mike Galbraith" <efault@....de>,
        "Rik van Riel" <riel@...hat.com>,
        "Frederic Weisbecker" <fweisbec@...il.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Wanpeng Li" <wanpeng.li@...mail.com>,
        "Radim Krčmář" <rkrcmar@...hat.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Ingo Molnar" <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: [PATCH 3.2 003/152] sched/cputime: Fix prev steal time accouting
 during CPU hotplug

3.2.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Wanpeng Li <wanpeng.li@...mail.com>

commit 3d89e5478bf550a50c99e93adf659369798263b0 upstream.

Commit:

  e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")

... set rq->prev_* to 0 after a CPU hotplug comes back, in order to
fix the case where (after CPU hotplug) steal time is smaller than
rq->prev_steal_time.

However, this should never happen. Steal time was only smaller because of the
KVM-specific bug fixed by the previous patch.  Worse, the previous patch
triggers a bug on CPU hot-unplug/plug operation: because
rq->prev_steal_time is cleared, all of the CPU's past steal time will be
accounted again on hot-plug.

Since the root cause has been fixed, we can just revert commit e9532e69b8d1.

Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Paolo Bonzini <pbonzini@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Fixes: 'commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")'
Link: http://lkml.kernel.org/r/1465813966-3116-3-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2084,19 +2084,6 @@ EXPORT_SYMBOL_GPL(account_system_vtime);
 
 #endif /* CONFIG_IRQ_TIME_ACCOUNTING */
 
-static inline void account_reset_rq(struct rq *rq)
-{
-#ifdef CONFIG_IRQ_TIME_ACCOUNTING
-	rq->prev_irq_time = 0;
-#endif
-#ifdef CONFIG_PARAVIRT
-	rq->prev_steal_time = 0;
-#endif
-#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
-	rq->prev_steal_time_rq = 0;
-#endif
-}
-
 #ifdef CONFIG_PARAVIRT
 static inline u64 steal_ticks(u64 steal)
 {
@@ -6869,7 +6856,6 @@ migration_call(struct notifier_block *nf
 
 	case CPU_UP_PREPARE:
 		rq->calc_load_update = calc_load_update;
-		account_reset_rq(rq);
 		break;
 
 	case CPU_ONLINE: