Commit f9c904b7 authored Mar 05, 2016 by Chris Friesen Committed by Ingo Molnar Mar 08, 2016

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>

parent 72f9f3fd

kernel/sched/cputime.c

+7 −7

Original line number	Diff line number	Diff line
		@@ -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;

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