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cputime: Fix jiffies based cputime assumption on steal accounting

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The steal guest time accounting code assumes that cputime_t is based on
jiffies. So when CONFIG_NO_HZ_FULL=y, which implies that cputime_t
is based on nsecs, steal_account_process_tick() passes the delta in
jiffies to account_steal_time() which then accounts it as if it's a
value in nsecs.

As a result, accounting 1 second of steal time (with HZ=100 that would
be 100 jiffies) is spuriously accounted as 100 nsecs.

As such /proc/stat may report 0 values of steal time even when two
guests have run concurrently for a few seconds on the same host and
same CPU.

In order to fix this, lets convert the nsecs based steal delta to
cputime instead of jiffies by using the right conversion API.

Given that the steal time is stored in cputime_t and this type can have
a smaller granularity than nsecs, we only account the rounded converted
value and leave the remaining nsecs for the next deltas.

Reported-by: Huiqingding <huding@redhat.com>
Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
This commit is contained in:
Frederic Weisbecker 2014-03-05 17:02:22 +01:00
parent d8a9ce3f8a
commit dee08a72de
1 changed files with 11 additions and 5 deletions
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16
kernel/sched/cputime.c
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@ -258,16 +258,22 @@ static __always_inline bool steal_account_process_tick(void)
{ {
#ifdef CONFIG_PARAVIRT #ifdef CONFIG_PARAVIRT
if (static_key_false(&paravirt_steal_enabled)) { if (static_key_false(&paravirt_steal_enabled)) {
u64 steal, st = 0; u64 steal;
cputime_t steal_ct;
steal = paravirt_steal_clock(smp_processor_id()); steal = paravirt_steal_clock(smp_processor_id());
steal -= this_rq()->prev_steal_time; steal -= this_rq()->prev_steal_time;
st = steal_ticks(steal); /*
this_rq()->prev_steal_time += st * TICK_NSEC; * cputime_t may be less precise than nsecs (eg: if it's
* based on jiffies). Lets cast the result to cputime
* 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);
account_steal_time(st); account_steal_time(steal_ct);
return st; return steal_ct;
} }
#endif #endif
return false; return false;