linux-kernel - [stable 4.4: PATCH] sched/cputime: Fix prev steal time accouting during CPU hotplug

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Date:   Mon, 31 Jul 2017 21:18:00 -0700
From:   Andres Oportus <andresoportus@...roid.com>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andres Oportus <andresoportus@...gle.com>
Subject: [stable 4.4: PATCH] sched/cputime: Fix prev steal time accouting during CPU hotplug

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

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Andres Oportus <andresoportus@...gle.com>
---
 kernel/sched/core.c  |  1 -
 kernel/sched/sched.h | 13 -------------
 2 files changed, 14 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c436426..b103b9c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5553,7 +5553,6 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 
 	case CPU_UP_PREPARE:
 		rq->calc_load_update = calc_load_update;
-		account_reset_rq(rq);
 		break;
 
 	case CPU_ONLINE:
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4e5db65..55d92a1 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1770,16 +1770,3 @@ static inline u64 irq_time_read(int cpu)
 }
 #endif /* CONFIG_64BIT */
 #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
-}
-- 
1.9.1