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SubjectRe: [PATCH] sched/cpufreq_schedutil: use now as reference when aggregating shared policy requests
On 3 May 2017 at 15:30, Juri Lelli <juri.lelli@arm.com> wrote:
> Currently, sugov_next_freq_shared() uses last_freq_update_time as a
> reference to decide when to start considering CPU contributions as
> stale.
>
> However, since last_freq_update_time is set by the last CPU that issued
> a frequency transition, this might cause problems in certain cases. In
> practice, the detection of stale utilization values fails whenever the
> CPU with such values was the last to update the policy. For example (and
> please note again that the SCHED_CPUFREQ_RT flag is not the problem
> here, but only the detection of after how much time that flag has to be
> considered stale), suppose a policy with 2 CPUs:
>
> CPU0 | CPU1
> |
> | RT task scheduled
> | SCHED_CPUFREQ_RT is set
> | CPU1->last_update = now
> | freq transition to max
> | last_freq_update_time = now
> |
>
> more than TICK_NSEC nsecs
>
> |
> a small CFS wakes up |
> CPU0->last_update = now1 |
> delta_ns(CPU0) < TICK_NSEC* |
> CPU0's util is considered |
> delta_ns(CPU1) = |
> last_freq_update_time - |
> CPU1->last_update = 0 |
> < TICK_NSEC |
> CPU1 is still considered |
> CPU1->SCHED_CPUFREQ_RT is set |
> we stay at max (until CPU1 |
> exits from idle) |
>
> * delta_ns is actually negative as now1 > last_freq_update_time
>
> While last_freq_update_time is a sensible reference for rate limiting,
> it doesn't seem to be useful for working around stale CPU states.
>
> Fix the problem by always considering now (time) as the reference for
> deciding when CPUs have stale contributions.
>
> Signed-off-by: Juri Lelli <juri.lelli@arm.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>

FWIW

Acked-by: Vincent Guittot <vincent.guittot@linaro.org>

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