Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 12 Oct 2023 17:01:54 +0200 | Subject | Re: [RFC PATCH] sched/fair: Bias runqueue selection towards almost idle prev CPU |
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On Thu, 12 Oct 2023 at 16:33, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> 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 >
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