Commit fb805d5e authored by Chris Friesen's avatar Chris Friesen Committed by Ben Hutchings

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

commit f9c904b7 upstream.

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: default avatarChris Friesen <chris.friesen@windriver.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
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.caSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
parent 6855d5ad
......@@ -259,21 +259,21 @@ 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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