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Message-ID: <56DBBDB8.40305@mail.usask.ca>
Date: Sat, 05 Mar 2016 23:18:48 -0600
From: Chris Friesen <cbf123@...l.usask.ca>
To: Frederic Weisbecker <fweisbec@...il.com>,
Thomas Gleixner <tglx@...utronix.de>
CC: John Stultz <john.stultz@...aro.org>,
Daniel Lezcano <daniel.lezcano@...aro.org>,
lkml <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: [PATCH v2] sched/cputime: steal_account_process_tick() should 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>
---
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 b2ab2ff..ab2b5fb 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(¶virt_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;
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