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

[Mathieu Desnoyers]

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 ?

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