linux-kernel - [tip:sched/core] sched/cputime: Fix steal_account_process

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Message-ID: <tip-f9c904b7613b8b4c85b10cd6b33ad41b2843fa9d@git.kernel.org>
Date:	Tue, 8 Mar 2016 05:18:07 -0800
From:	tip-bot for Chris Friesen <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	cbf123@...l.usask.ca, mingo@...nel.org, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	chris.friesen@...driver.com, hpa@...or.com, fweisbec@...il.com
Subject: [tip:sched/core] sched/cputime: Fix steal_account_process_tick() to
 always return jiffies

Commit-ID:  f9c904b7613b8b4c85b10cd6b33ad41b2843fa9d
Gitweb:     http://git.kernel.org/tip/f9c904b7613b8b4c85b10cd6b33ad41b2843fa9d
Author:     Chris Friesen <cbf123@...l.usask.ca>
AuthorDate: Sat, 5 Mar 2016 23:18:48 -0600
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 8 Mar 2016 12:24:56 +0100

sched/cputime: Fix steal_account_process_tick() to always return jiffies

The callers of steal_account_process_tick() expect it to return
whether a jiffy should be considered stolen or not.

Currently the return value of steal_account_process_tick() is in
units of cputime, which vary between either jiffies or nsecs
depending on CONFIG_VIRT_CPU_ACCOUNTING_GEN.

If cputime has nsecs granularity and there is a tiny amount of
stolen time (a few nsecs, say) then we will consider the entire
tick stolen and will not account the tick on user/system/idle,
causing /proc/stats to show invalid data.

The fix is to change steal_account_process_tick() to accumulate
the stolen time and only account it once it's worth a jiffy.

(Thanks to Frederic Weisbecker for suggestions to fix a bug in my
first version of the patch.)

Signed-off-by: Chris Friesen <chris.friesen@...driver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: <stable@...r.kernel.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/56DBBDB8.40305@mail.usask.ca
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/cputime.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 01d9898..75f98c5 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -262,21 +262,21 @@ static __always_inline bool steal_account_process_tick(void)
 #ifdef CONFIG_PARAVIRT
 	if (static_key_false(&paravirt_steal_enabled)) {
 		u64 steal;
-		cputime_t steal_ct;
+		unsigned long steal_jiffies;
 
 		steal = paravirt_steal_clock(smp_processor_id());
 		steal -= this_rq()->prev_steal_time;
 
 		/*
-		 * cputime_t may be less precise than nsecs (eg: if it's
-		 * based on jiffies). Lets cast the result to cputime
+		 * steal is in nsecs but our caller is expecting steal
+		 * time in jiffies. Lets cast the result to jiffies
 		 * granularity and account the rest on the next rounds.
 		 */
-		steal_ct = nsecs_to_cputime(steal);
-		this_rq()->prev_steal_time += cputime_to_nsecs(steal_ct);
+		steal_jiffies = nsecs_to_jiffies(steal);
+		this_rq()->prev_steal_time += jiffies_to_nsecs(steal_jiffies);
 
-		account_steal_time(steal_ct);
-		return steal_ct;
+		account_steal_time(jiffies_to_cputime(steal_jiffies));
+		return steal_jiffies;
 	}
 #endif
 	return false;