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

[Vincent Guittot]

On Thu, 12 Oct 2023 at 16:33, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> On 2023-10-11 06:16, Chen Yu wrote:
> > On 2023-10-10 at 09:49:54 -0400, Mathieu Desnoyers 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 ?
> >>
> >
> > My understanding is that, the load_sum of a single task is a value in "us" out
> > of a window of 1000 us, while the load_avg of the task will multiply the weight
> > of the task. In this case a task with nice 0 is NICE_0_WEIGHT = 1024.
> >
> > __update_load_avg_se -> ___update_load_sum calculate the load_sum of a task(there
> > is comments around ___update_load_sum to describe the pelt calculation),
> > and ___update_load_avg() calculate the load_avg based on the task's weight.
>
> Thanks for your thorough explanation, now it makes sense.
>
> I understand as well that the cfs_rq->avg.load_sum is the result of summing
> each task load_sum multiplied by their weight:

Please don't use load_sum but only *_avg.
As already said, util_avg or runnable_avg are better metrics for you

>
> static inline void
> enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
>          cfs_rq->avg.load_avg += se->avg.load_avg;
>          cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
> }
>
> Therefore I think we need to multiply the load_sum value we aim for by
> get_pelt_divider(&cpu_rq(cpu)->cfs.avg) to compare it to a rq load_sum.
>
> I plan to compare the rq load sum to "10 * get_pelt_divider(&cpu_rq(cpu)->cfs.avg)"
> to match runqueues which were previously idle (therefore with prior periods contribution
> to the rq->load_sum being pretty much zero), and which have a current period rq load_sum
> below or equal 10us per 1024us (<= 1%):
>
> static inline unsigned long cfs_rq_weighted_load_sum(struct cfs_rq *cfs_rq)
> {
>          return cfs_rq->avg.load_sum;
> }
>
> static unsigned long cpu_weighted_load_sum(struct rq *rq)
> {
>          return cfs_rq_weighted_load_sum(&rq->cfs);
> }
>
> /*
>   * A runqueue is considered almost idle if:
>   *
>   *   cfs_rq->avg.load_sum / get_pelt_divider(&cfs_rq->avg) / 1024 <= 1%
>   *
>   * This inequality is transformed as follows to minimize arithmetic:
>   *
>   *   cfs_rq->avg.load_sum <= get_pelt_divider(&cfs_rq->avg) * 10
>   */
> static bool
> almost_idle_cpu(int cpu, struct task_struct *p)
> {
>          if (!sched_feat(WAKEUP_BIAS_PREV_IDLE))
>                  return false;
>          return cpu_weighted_load_sum(cpu_rq(cpu)) <= 10 * get_pelt_divider(&cpu_rq(cpu)->cfs.avg);
> }
>
> Does it make sense ?
>
> Thanks,
>
> Mathieu
>
>
> >
> >> 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 ?
> >>
> >
> > For hackbench -g 16 -f 20 --pipe  -l 480000 -s 100, it is around 8~10% idle
> > For hackbench -g 16 -f 20   -l 480000 -s 100, it is around 2~3% idle
> >
> > Then the CPUs in packge 1 are offlined to get stable result when the group number is low.
> > hackbench -g 1 -f 20 --pipe  -l 480000 -s 100
> > Some CPUs are busy, others are idle, and some are half-busy.
> > Core  CPU     Busy%
> > -     -       49.57
> > 0     0       1.89
> > 0     72      75.55
> > 1     1       100.00
> > 1     73      0.00
> > 2     2       100.00
> > 2     74      0.00
> > 3     3       100.00
> > 3     75      0.01
> > 4     4       78.29
> > 4     76      17.72
> > 5     5       100.00
> > 5     77      0.00
> >
> >
> > hackbench -g 1 -f 20  -l 480000 -s 100
> > Core  CPU     Busy%
> > -     -       48.29
> > 0     0       57.94
> > 0     72      21.41
> > 1     1       83.28
> > 1     73      0.00
> > 2     2       11.44
> > 2     74      83.38
> > 3     3       21.45
> > 3     75      77.27
> > 4     4       26.89
> > 4     76      80.95
> > 5     5       5.01
> > 5     77      83.09
> >
> >
> > echo NO_WAKEUP_BIAS_PREV_IDLE > /sys/kernel/debug/sched/features
> > hackbench -g 1 -f 20 --pipe  -l 480000 -s 100
> > Running in process mode with 1 groups using 40 file descriptors each (== 40 tasks)
> > Each sender will pass 480000 messages of 100 bytes
> > Time: 9.434
> >
> > echo WAKEUP_BIAS_PREV_IDLE > /sys/kernel/debug/sched/features
> > hackbench -g 1 -f 20 --pipe  -l 480000 -s 100
> > Running in process mode with 1 groups using 40 file descriptors each (== 40 tasks)
> > Each sender will pass 480000 messages of 100 bytes
> > Time: 9.373
> >
> > thanks,
> > Chenyu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>