Re: [PATCH 4/6] cputime: Fix jiffies based cputime assumption on steal accounting

[Rik van Riel <riel@redhat.com>]

On 03/06/2014 12:56 PM, Frederic Weisbecker wrote:
> 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>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

Acked-by: Rik van Riel <riel@redhat.com>