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

[Chen Yu]

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.
 
> 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