On Wednesday, May 03, 2017 02:30:48 PM Juri Lelli 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 <[email protected]>
> Cc: Rafael J. Wysocki <[email protected]>
> Cc: Viresh Kumar <[email protected]>
OK
I'll queue this up if there are no objections from the people in the CC.
> ---
> kernel/sched/cpufreq_schedutil.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c
> b/kernel/sched/cpufreq_schedutil.c
> index 76877a62b5fa..622eed1b7658 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -245,11 +245,10 @@ static void sugov_update_single(struct update_util_data
> *hook, u64 time,
> sugov_update_commit(sg_policy, time, next_f);
> }
>
> -static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu)
> +static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64
> time)
> {
> struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> struct cpufreq_policy *policy = sg_policy->policy;
> - u64 last_freq_update_time = sg_policy->last_freq_update_time;
> unsigned long util = 0, max = 1;
> unsigned int j;
>
> @@ -265,7 +264,7 @@ static unsigned int sugov_next_freq_shared(struct
> sugov_cpu *sg_cpu)
> * enough, don't take the CPU into account as it probably is
> * idle now (and clear iowait_boost for it).
> */
> - delta_ns = last_freq_update_time - j_sg_cpu->last_update;
> + delta_ns = time - j_sg_cpu->last_update;
> if (delta_ns > TICK_NSEC) {
> j_sg_cpu->iowait_boost = 0;
> continue;
> @@ -309,7 +308,7 @@ static void sugov_update_shared(struct update_util_data
> *hook, u64 time,
> if (flags & SCHED_CPUFREQ_RT_DL)
> next_f = sg_policy->policy->cpuinfo.max_freq;
> else
> - next_f = sugov_next_freq_shared(sg_cpu);
> + next_f = sugov_next_freq_shared(sg_cpu, time);
>
> sugov_update_commit(sg_policy, time, next_f);
> }
>
Thanks,
Rafael
