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

[Chen Yu]

On 2023-10-12 at 17:26:36 +0200, Vincent Guittot wrote:
> On Wed, 11 Oct 2023 at 12:17, Chen Yu <yu.c.chen@xxxxxxxxx> 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
> 
> I'm not sure we can say this. We use a 1024us sampling rate for
> calculating weighted average but load_sum is in the range [0:47742]
> so what does it mean 47742us out of a window of 1000us ?
> 
> Beside this we have util_avg in the range [0:cpu capacity] which gives
> you the average running time of the cpu
>

Sorry I did not describe it accurately. Yes, it should be 1024us instead of 1000us.
And the load_sum is the decayed accumulated duration. util_avg was once used
previously and Mathieu found that it did not work. But in the latest version
it works again, I'll have a test on that version.

thanks,
Chenyu