Re: [PATCH v2] cpufreq: governor: Be friendly towards latency-sensitive bursty workloads

From: Rafael J. Wysocki
Date: Fri Jun 06 2014 - 17:19:59 EST

On Wednesday, June 04, 2014 03:17:00 AM Srivatsa S. Bhat wrote:
> Cpufreq governors like the ondemand governor calculate the load on the CPU
> periodically by employing deferrable timers. A deferrable timer won't fire
> if the CPU is completely idle (and there are no other timers to be run), in
> order to avoid unnecessary wakeups and thus save CPU power.
>
> However, the load calculation logic is agnostic to all this, and this can
> lead to the problem described below.
>
>
> Time (ms) CPU 1
>
> 100 Task-A running
>
> 110 Governor's timer fires, finds load as 100% in the last
> 10ms interval and increases the CPU frequency.
>
> 110.5 Task-A running
>
> 120 Governor's timer fires, finds load as 100% in the last
> 10ms interval and increases the CPU frequency.
>
> 125 Task-A went to sleep. With nothing else to do, CPU 1
> went completely idle.
>
> 200 Task-A woke up and started running again.
>
> 200.5 Governor's deferred timer (which was originally programmed
> to fire at time 130) fires now. It calculates load for the
> time period 120 to 200.5, and finds the load is almost zero.
> Hence it decreases the CPU frequency to the minimum.
>
> 210 Governor's timer fires, finds load as 100% in the last
> 10ms interval and increases the CPU frequency.
>
>
> So, after the workload woke up and started running, the frequency was suddenly
> dropped to absolute minimum, and after that, there was an unnecessary delay of
> 10ms (sampling period) to increase the CPU frequency back to a reasonable value.
> And this pattern repeats for every wake-up-from-cpu-idle for that workload.
> This can be quite undesirable for latency- or response-time sensitive bursty
> workloads. So we need to fix the governor's logic to detect such wake-up-from-
> cpu-idle scenarios and start the workload at a reasonably high CPU frequency.
>
> One extreme solution would be to fake a load of 100% in such scenarios. But
> that might lead to undesirable side-effects such as frequency spikes (which
> might also need voltage changes) especially if the previous frequency happened
> to be very low.
>
> We just want to avoid the stupidity of dropping down the frequency to a minimum
> and then enduring a needless (and long) delay before ramping it up back again.
> So, let us simply carry forward the previous load - that is, let us just pretend
> that the 'load' for the current time-window is the same as the load for the
> previous window. That way, the frequency and voltage will continue to be set
> to whatever values they were set at previously. This means that bursty workloads
> will get a chance to influence the CPU frequency at which they wake up from
> cpu-idle, based on their past execution history. Thus, they might be able to
> avoid suffering from slow wakeups and long response-times.
>
> [ The right way to solve this problem is to teach the CPU frequency governors
> to track load on a per-task basis, not a per-CPU basis, and set the appropriate
> frequency on whichever CPU the task executes. But that involves redesigning
> the cpufreq subsystem, so this patch should make the situation bearable until
> then. ]
>
> Experimental results:
> ====================

This formatting of the changelog evidently confused Patchwork.

That's not a big deal, but please try to avoid that in the future if possible.

Rafael


>
> I ran a modified version of ebizzy (called 'sleeping-ebizzy') that sleeps in
> between its execution such that its total utilization can be a user-defined
> value, say 10% or 20% (higher the utilization specified, lesser the amount of
> sleeps injected). This ebizzy was run with a single-thread, tied to CPU 8.
>
> Behavior observed with tracing (sample taken from 40% utilization runs):
> ------------------------------------------------------------------------
>
> Without patch:
> ~~~~~~~~~~~~~~
> kworker/8:2-12137 416.335742: cpu_frequency: state=2061000 cpu_id=8
> kworker/8:2-12137 416.335744: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40753 416.345741: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> kworker/8:2-12137 416.345744: cpu_frequency: state=4123000 cpu_id=8
> kworker/8:2-12137 416.345746: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40753 416.355738: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> <snip> --------------------------------------------------------------------- <snip>
> <...>-40753 416.402202: sched_switch: prev_comm=ebizzy ==> next_comm=swapper/8
> <idle>-0 416.502130: sched_switch: prev_comm=swapper/8 ==> next_comm=ebizzy
> <...>-40753 416.505738: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> kworker/8:2-12137 416.505739: cpu_frequency: state=2061000 cpu_id=8
> kworker/8:2-12137 416.505741: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40753 416.515739: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> kworker/8:2-12137 416.515742: cpu_frequency: state=4123000 cpu_id=8
> kworker/8:2-12137 416.515744: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
>
> Observation: Ebizzy went idle at 416.402202, and started running again at
> 416.502130. But cpufreq noticed the long idle period, and dropped the frequency
> at 416.505739, only to increase it back again at 416.515742, realizing that the
> workload is in-fact CPU bound. Thus ebizzy needlessly ran at the lowest frequency
> for almost 13 milliseconds (almost 1 full sample period), and this pattern
> repeats on every sleep-wakeup. This could hurt latency-sensitive workloads quite
> a lot.
>
> With patch:
> ~~~~~~~~~~~
>
> kworker/8:2-29802 464.832535: cpu_frequency: state=2061000 cpu_id=8
> <snip> --------------------------------------------------------------------- <snip>
> kworker/8:2-29802 464.962538: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40738 464.972533: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> kworker/8:2-29802 464.972536: cpu_frequency: state=4123000 cpu_id=8
> kworker/8:2-29802 464.972538: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40738 464.982531: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> <snip> --------------------------------------------------------------------- <snip>
> kworker/8:2-29802 465.022533: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40738 465.032531: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> kworker/8:2-29802 465.032532: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40738 465.035797: sched_switch: prev_comm=ebizzy ==> next_comm=swapper/8
> <idle>-0 465.240178: sched_switch: prev_comm=swapper/8 ==> next_comm=ebizzy
> <...>-40738 465.242533: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
> kworker/8:2-29802 465.242535: sched_switch: prev_comm=kworker/8:2 ==> next_comm=ebizzy
> <...>-40738 465.252531: sched_switch: prev_comm=ebizzy ==> next_comm=kworker/8:2
>
> Observation: Ebizzy went idle at 465.035797, and started running again at
> 465.240178. Since ebizzy was the only real workload running on this CPU,
> cpufreq retained the frequency at 4.1Ghz throughout the run of ebizzy, no
> matter how many times ebizzy slept and woke-up in-between. Thus, ebizzy
> got the 10ms worth of 4.1 Ghz benefit during every sleep-wakeup (as compared
> to the run without the patch) and this boost gave a modest improvement in total
> throughput, as shown below.
>
> Sleeping-ebizzy records-per-second:
> -----------------------------------
>
> Utilization Without patch With patch Difference (Absolute and % values)
> 10% 274767 277046 + 2279 (+0.829%)
> 20% 543429 553484 + 10055 (+1.850%)
> 40% 1090744 1107959 + 17215 (+1.578%)
> 60% 1634908 1662018 + 27110 (+1.658%)
>
> A rudimentary and somewhat approximately latency-sensitive workload such as
> sleeping-ebizzy itself showed a consistent, noticeable performance improvement
> with this patch. Hence, workloads that are truly latency-sensitive will benefit
> quite a bit from this change. Moreover, this is an overall win-win since this
> patch does not hurt power-savings at all (because, this patch does not reduce
> the idle time or idle residency; and the high frequency of the CPU when it goes
> to cpu-idle does not affect/hurt the power-savings of deep idle states).
>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Gautham R. Shenoy <ego@xxxxxxxxxxxxxxxxxx>
> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> ---
>
> Changes in v2:
> * Removed the 'sampling_rate' parameter to dbs_check_cpu() to make the code
> cleaner, as suggested by Viresh.
>
> drivers/cpufreq/cpufreq_governor.c | 47 ++++++++++++++++++++++++++++++++++--
> drivers/cpufreq/cpufreq_governor.h | 1 +
> 2 files changed, 45 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> index e1c6433..2597bbe 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -36,14 +36,29 @@ void dbs_check_cpu(struct dbs_data *dbs_data, int cpu)
> struct od_dbs_tuners *od_tuners = dbs_data->tuners;
> struct cs_dbs_tuners *cs_tuners = dbs_data->tuners;
> struct cpufreq_policy *policy;
> + unsigned int sampling_rate;
> unsigned int max_load = 0;
> unsigned int ignore_nice;
> unsigned int j;
>
> - if (dbs_data->cdata->governor == GOV_ONDEMAND)
> + if (dbs_data->cdata->governor == GOV_ONDEMAND) {
> + struct od_cpu_dbs_info_s *od_dbs_info =
> + dbs_data->cdata->get_cpu_dbs_info_s(cpu);
> +
> + /*
> + * Sometimes, the ondemand governor uses an additional
> + * multiplier to give long delays. So apply this multiplier to
> + * the 'sampling_rate', so as to keep the wake-up-from-idle
> + * detection logic a bit conservative.
> + */
> + sampling_rate = od_tuners->sampling_rate;
> + sampling_rate *= od_dbs_info->rate_mult;
> +
> ignore_nice = od_tuners->ignore_nice_load;
> - else
> + } else {
> + sampling_rate = cs_tuners->sampling_rate;
> ignore_nice = cs_tuners->ignore_nice_load;
> + }
>
> policy = cdbs->cur_policy;
>
> @@ -96,7 +111,29 @@ void dbs_check_cpu(struct dbs_data *dbs_data, int cpu)
> if (unlikely(!wall_time || wall_time < idle_time))
> continue;
>
> - load = 100 * (wall_time - idle_time) / wall_time;
> + /*
> + * If the CPU had gone completely idle, and a task just woke up
> + * on this CPU now, it would be unfair to calculate 'load' the
> + * usual way for this elapsed time-window, because it will show
> + * near-zero load, irrespective of how CPU intensive the new
> + * task is. This is undesirable for latency-sensitive bursty
> + * workloads.
> + *
> + * To avoid this, we reuse the 'load' from the previous
> + * time-window and give this task a chance to start with a
> + * reasonably high CPU frequency.
> + *
> + * Detecting this situation is easy: the governor's deferrable
> + * timer would not have fired during CPU-idle periods. Hence
> + * an unusually large 'wall_time' (as compared to the sampling
> + * rate) indicates this scenario.
> + */
> + if (unlikely(wall_time > (2 * sampling_rate))) {
> + load = j_cdbs->prev_load;
> + } else {
> + load = 100 * (wall_time - idle_time) / wall_time;
> + j_cdbs->prev_load = load;
> + }
>
> if (load > max_load)
> max_load = load;
> @@ -323,6 +360,10 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> j_cdbs->cur_policy = policy;
> j_cdbs->prev_cpu_idle = get_cpu_idle_time(j,
> &j_cdbs->prev_cpu_wall, io_busy);
> + j_cdbs->prev_load = 100 * (j_cdbs->prev_cpu_wall -
> + j_cdbs->prev_cpu_idle) /
> + j_cdbs->prev_cpu_wall;
> +
> if (ignore_nice)
> j_cdbs->prev_cpu_nice =
> kcpustat_cpu(j).cpustat[CPUTIME_NICE];
> diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
> index bfb9ae1..b56552b 100644
> --- a/drivers/cpufreq/cpufreq_governor.h
> +++ b/drivers/cpufreq/cpufreq_governor.h
> @@ -134,6 +134,7 @@ struct cpu_dbs_common_info {
> u64 prev_cpu_idle;
> u64 prev_cpu_wall;
> u64 prev_cpu_nice;
> + unsigned int prev_load;
> struct cpufreq_policy *cur_policy;
> struct delayed_work work;
> /*
>
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--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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