sched/cputime: Fix steal_account_process_tick() to always return jiffies
authorChris Friesen <cbf123@mail.usask.ca>
Sun, 6 Mar 2016 05:18:48 +0000 (23:18 -0600)
committerIngo Molnar <mingo@kernel.org>
Tue, 8 Mar 2016 11:24:56 +0000 (12:24 +0100)
commitf9c904b7613b8b4c85b10cd6b33ad41b2843fa9d
tree96f16153b74572cf26a98c8028ffe315f5cbafe7
parent72f9f3fdc928dc3ecd223e801b32d930b662b6ed
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@windriver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/56DBBDB8.40305@mail.usask.ca
Signed-off-by: Ingo Molnar <mingo@kernel.org>
kernel/sched/cputime.c