Re: [RFC PATCH 3/3] sched/fair: Add a per-shard overload flag

[Chen Yu]

Hi Prateek,

On 2023-09-27 at 09:53:13 +0530, K Prateek Nayak wrote:
> Hello David,
> 
> Some more test results (although this might be slightly irrelevant with
> next version around the corner)
> 
> On 9/1/2023 12:41 AM, David Vernet wrote:
> > On Thu, Aug 31, 2023 at 04:15:08PM +0530, K Prateek Nayak wrote:
> > 
> -> With EEVDF
> 
> o tl;dr
> 
> - Same as what was observed without EEVDF  but shared_runq shows
>   serious regression with multiple more variants of tbench and
>   netperf now.
> 
> o Kernels
> 
> eevdf			: tip:sched/core at commit b41bbb33cf75 ("Merge branch 'sched/eevdf' into sched/core")
> shared_runq		: eevdf + correct time accounting with v3 of the series without any other changes
> shared_runq_idle_check	: shared_runq + move the rq->avg_idle check before peeking into the shared_runq
> 			  (the rd->overload check still remains below the shared_runq access)
>

I did not see any obvious regression on a Sapphire Rapids server and it seems that
the result on your platform suggests that C/S workload could be impacted
by shared_runq. Meanwhile some individual workloads like HHVM in David's environment
(no shared resource between tasks if I understand correctly) could benefit from
shared_runq a lot. This makes me wonder if we can let shared_runq skip the C/S tasks.
The question would be how to define C/S tasks. At first thought:
A only wakes up B, and B only wakes up A, then they could be regarded as a pair
of C/S
 (A->last_wakee == B && B->last_wakee == A &&
  A->wakee_flips <= 1 && B->wakee_flips <= 1)
But for netperf/tbench, this does not apply, because netperf client leverages kernel
thread(workqueue) to wake up the netserver, that is A wakes up kthread T, then T
wakes up B. Unless we have a chain, we can not detect this wakeup behavior.

thanks,
Chenyu