Date:	Sat, 07 Jun 2014 23:13:35 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	viresh.kumar@...aro.org, pavel@....cz, svaidy@...ux.vnet.ibm.com,
	ego@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] cpufreq: governor: Be friendly towards latency-sensitive bursty workloads

On Sunday, June 08, 2014 02:11:43 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.
> 
> However, we should take care not to over-do this. For example, such a "copy
> previous load" logic will benefit cases like this: (where # represents busy
> and . represents idle)
> 
> ##########.........#########.........###########...........##########........
> 
> but it will be detrimental in cases like the one shown below, because it will
> retain the high frequency (copied from the previous interval) even in a mostly
> idle system:
> 
> ##########.........#.................#.....................#...............
> 
> (i.e., the workload finished and the remaining tasks are such that their busy
> periods are smaller than the sampling interval, which causes the timer to
> always get deferred. So, this will make the copy-previous-load logic copy
> the initial high load to subsequent idle periods over and over again, thus
> keeping the frequency high unnecessarily).
> 
> So, we modify this copy-previous-load logic such that it is used only once
> upon every wakeup-from-idle. Thus if we have 2 consecutive idle periods, the
> previous load won't get blindly copied over; cpufreq will freshly evaluate the
> load in the second idle interval, thus ensuring that the system comes back to
> its normal state.
> 
> [ The right way to solve this whole problem is to teach the CPU frequency
> governors to also track load on a per-task basis, not just a per-CPU basis,
> and then use both the data sources intelligently to set the appropriate
> frequency on the CPUs. But that involves redesigning the cpufreq subsystem,
> so this patch should make the situation bearable until then. ]
> 
> Experimental results:
> +-------------------+
> 
> 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@...ux.vnet.ibm.com>
> Reviewed-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> 
> Changes in v3:
> * Modified the "copy-previous-load" logic to copy only once, upon the first
>   wakeup from idle, to fix the flaw pointed out by Pavel Machek.
> 
> * Fixed the 64 bit division issue reported by Fengguang Wu's build robot.

Applied to bleeding-edge, thanks!


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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