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 <[email protected]> > Reviewed-by: Gautham R. Shenoy <[email protected]> > Acked-by: Viresh Kumar <[email protected]> > --- > > 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; > /* > > -- > To unsubscribe from this list: send the line "unsubscribe linux-pm" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
