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

[Chen Yu]

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

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.

thanks,
Chenyu