Re: [RFC PATCH] sched/fair: Bias runqueue selection towards almost idle prev CPU

[Vincent Guittot]

On Tue, 10 Oct 2023 at 15:49, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> On 2023-10-09 01:14, Chen Yu wrote:
> > On 2023-09-30 at 07:45:38 -0400, Mathieu Desnoyers wrote:
> >> On 9/30/23 03:11, Chen Yu wrote:
> >>> Hi Mathieu,
> >>>
> >>> On 2023-09-29 at 14:33:50 -0400, Mathieu Desnoyers wrote:
> >>>> Introduce the WAKEUP_BIAS_PREV_IDLE scheduler feature. It biases
> >>>> select_task_rq towards the previous CPU if it was almost idle
> >>>> (avg_load <= 0.1%).
> >>>
> >>> Yes, this is a promising direction IMO. One question is that,
> >>> can cfs_rq->avg.load_avg be used for percentage comparison?
> >>> If I understand correctly, load_avg reflects that more than
> >>> 1 tasks could have been running this runqueue, and the
> >>> load_avg is the direct proportion to the load_weight of that
> >>> cfs_rq. Besides, LOAD_AVG_MAX seems to not be the max value
> >>> that load_avg can reach, it is the sum of
> >>> 1024 * (y + y^1 + y^2 ... )
> >>>
> >>> For example,
> >>> taskset -c 1 nice -n -20 stress -c 1
> >>> cat /sys/kernel/debug/sched/debug | grep 'cfs_rq\[1\]' -A 12 | grep "\.load_avg"
> >>>     .load_avg                      : 88763
> >>>     .load_avg                      : 1024
> >>>
> >>> 88763 is higher than LOAD_AVG_MAX=47742
> >>
> >> I would have expected the load_avg to be limited to LOAD_AVG_MAX somehow,
> >> but it appears that it does not happen in practice.
> >>
> >> That being said, if the cutoff is really at 0.1% or 0.2% of the real max,
> >> does it really matter ?
> >>
> >>> Maybe the util_avg can be used for precentage comparison I suppose?
> >> [...]
> >>> Or
> >>> return cpu_util_without(cpu_rq(cpu), p) * 1000 <= capacity_orig_of(cpu) ?
> >>
> >> Unfortunately using util_avg does not seem to work based on my testing.
> >> Even at utilization thresholds at 0.1%, 1% and 10%.
> >>
> >> Based on comments in fair.c:
> >>
> >>   * CPU utilization is the sum of running time of runnable tasks plus the
> >>   * recent utilization of currently non-runnable tasks on that CPU.
> >>
> >> I think we don't want to include currently non-runnable tasks in the
> >> statistics we use, because we are trying to figure out if the cpu is a
> >> idle-enough target based on the tasks which are currently running, for the
> >> purpose of runqueue selection when waking up a task which is considered at
> >> that point in time a non-runnable task on that cpu, and which is about to
> >> become runnable again.
> >>
> >
> > Although LOAD_AVG_MAX is not the max possible load_avg, we still want to find
> > a proper threshold to decide if the CPU is almost idle. The LOAD_AVG_MAX
> > based threshold is modified a little bit:
> >
> > The theory is, if there is only 1 task on the CPU, and that task has a nice
> > of 0, the task runs 50 us every 1000 us, then this CPU is regarded as almost
> > idle.
> >
> > The load_sum of the task is:
> > 50 * (1 + y + y^2 + ... + y^n)
> > The corresponding avg_load of the task is approximately
> > NICE_0_WEIGHT * load_sum / LOAD_AVG_MAX = 50.
> > So:
> >
> > /* which is close to LOAD_AVG_MAX/1000 = 47 */
> > #define ALMOST_IDLE_CPU_LOAD   50
>
> Sorry to be slow at understanding this concept, but this whole "load"
> value is still somewhat magic to me.
>
> Should it vary based on CONFIG_HZ_{100,250,300,1000}, or is it
> independent ? Where is it documented that the load is a value in "us"
> out of a window of 1000 us ?

nowhere because load_avg is not in usec. load_avg is the sum of
entities' load_avg which is based on the weight of the entity. The
weight of an entity is in the range [2:88761] and as a result its
load_avg. LOAD_AVG_MAX can be used with the *_sum fields but not the
*_avg fields of struct sched_avg

If you want to evaluate the idleness of a CPU with pelt signal, you
should better use util_avg or runnable_avg which are unweighted values
in the range [0:1024]



>
> And with this value "50", it would cover the case where there is only a
> single task taking less than 50us per 1000us, and cases where the sum
> for the set of tasks on the runqueue is taking less than 50us per 1000us
> overall.
>
> >
> > static bool
> > almost_idle_cpu(int cpu, struct task_struct *p)
> > {
> >         if (!sched_feat(WAKEUP_BIAS_PREV_IDLE))
> >                 return false;
> >         return cpu_load_without(cpu_rq(cpu), p) <= ALMOST_IDLE_CPU_LOAD;
> > }
> >
> > Tested this on Intel Xeon Platinum 8360Y, Ice Lake server, 36 core/package,
> > total 72 core/144 CPUs. Slight improvement is observed in hackbench socket mode:
> >
> > socket mode:
> > hackbench -g 16 -f 20 -l 480000 -s 100
> >
> > Before patch:
> > Running in process mode with 16 groups using 40 file descriptors each (== 640 tasks)
> > Each sender will pass 480000 messages of 100 bytes
> > Time: 81.084
> >
> > After patch:
> > Running in process mode with 16 groups using 40 file descriptors each (== 640 tasks)
> > Each sender will pass 480000 messages of 100 bytes
> > Time: 78.083
> >
> >
> > pipe mode:
> > hackbench -g 16 -f 20 --pipe  -l 480000 -s 100
> >
> > Before patch:
> > Running in process mode with 16 groups using 40 file descriptors each (== 640 tasks)
> > Each sender will pass 480000 messages of 100 bytes
> > Time: 38.219
> >
> > After patch:
> > Running in process mode with 16 groups using 40 file descriptors each (== 640 tasks)
> > Each sender will pass 480000 messages of 100 bytes
> > Time: 38.348
> >
> > It suggests that, if the workload has larger working-set/cache footprint, waking up
> > the task on its previous CPU could get more benefit.
>
> In those tests, what is the average % of idleness of your cpus ?
>
> Thanks,
>
> Mathieu
>
> >
> > thanks,
> > Chenyu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>