Re: [PATCH 1/2] sched: Optimise task_mm_cid_work duration

[Gabriele Monaco <gmonaco@redhat.com>]

Hi Mathieu,

thanks for the quick reply.

> Thanks for looking into this. I understand that you are after
> minimizing the
> latency introduced by task_mm_cid_work on isolated cores. I think
> we'll need
> to think a bit harder, because the proposed solution does not work:
> 
>   * for_each_cpu_from - iterate over CPUs present in @mask, from @cpu
> to the end of @mask.
> 
> cpu is uninitialized. So this is completely broken. 

My bad, wrong macro.. Should be for_each_cpu

> Was this tested
> against a workload that actually uses concurrency IDs to ensure it
> does
> not break the whole thing ? Did you run the rseq selftests ?
> 

I did run the stress-ng --rseq command for a while and didn't see any
error reported, but it's probably not bulletproof. I'll use the
selftests for the next iterations.

> Also, the mm_cidmask is a mask of concurrency IDs, not a mask of
> CPUs. So
> using it to iterate on CPUs is wrong.
> 

Mmh I get it, during my tests I was definitely getting better results
than using the mm_cpus_allowed mask, but I guess that was a broken test
so it just doesn't count..
Do you think using mm_cpus_allowed would make more sense, with the
/risk/ of being a bit over-cautious?


Gabriele