linux-kernel - [FIX][PATCH 01/10] powerpc32: Fix stale scaled stime on context switch

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Date:   Tue,  6 Dec 2016 03:32:14 +0100
From:   Frederic Weisbecker <fweisbec@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Mackerras <paulus@...ba.org>,
        Ingo Molnar <mingo@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Rik van Riel <riel@...hat.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>
Subject: [FIX][PATCH 01/10] powerpc32: Fix stale scaled stime on context switch

On context switch with powerpc32, the cputime is accumulated in the
thread_info struct. So the switching-in task must move forward its
start time snapshot to the current time in order to later compute the
delta spent in system mode.

This is what we do for the normal cputime by initializing the starttime
field to the value of the previous task's starttime which got freshly
updated.

But we are missing the update of the scaled cputime start time. As a
result we may be accounting too much scaled cputime later.

Fix this by initializing the scaled cputime the same way we do for
normal cputime.

Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Heiko Carstens <heiko.carstens@...ibm.com>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Tony Luck <tony.luck@...el.com>
Cc: Fenghua Yu <fenghua.yu@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Stanislaw Gruszka <sgruszka@...hat.com>
Cc: Wanpeng Li <wanpeng.li@...mail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 arch/powerpc/kernel/time.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index be9751f..bca2781 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -407,6 +407,7 @@ void arch_vtime_task_switch(struct task_struct *prev)
 	struct cpu_accounting_data *acct = get_accounting(current);
 
 	acct->starttime = get_accounting(prev)->starttime;
+	acct->startspurr = get_accounting(prev)->startspurr;
 	acct->system_time = 0;
 	acct->user_time = 0;
 }
-- 
2.7.4