Re: [PATCH v2 00/12] sched: Address schbench regression

[Chris Mason]

On 7/22/25 1:20 PM, Chris Mason wrote:
> On 7/21/25 12:37 PM, Shrikanth Hegde wrote:
> 
>> *** issues/observations in schbench.
>>
>> Chris,
>>
>> When one does -W auto or -M auto i think code is meant to run, n message
>> threads on first n CPUs and worker threads
>> on remaining CPUs?
>> I don't see that happening.  above behavior can be achieved only with -M
>> <cpus> -W <cpus>
>>
>>         int i = 0;
>>         CPU_ZERO(m_cpus);
>>         for (int i = 0; i < m_threads; ++i) {
>>                 CPU_SET(i, m_cpus);
>>                 CPU_CLR(i, w_cpus);
>>         }
>>         for (; i < CPU_SETSIZE; i++) {             << here i refers to
>> the one in scope. which is 0. Hence w_cpus is set for all cpus.
>>                                                       And hence workers
>> end up running on all CPUs even with -W auto
>>                 CPU_SET(i, w_cpus);
>>         }
> 
> Oh, you're exactly right.  Fixing this up, thanks.  I'll do some runs to
> see if this changes things on my test boxes as well.

Fixing this makes it substantially slower (5.2M RPS -> 3.8M RPS), with
more time spent in select_task_rq().  I need to trace a bit to
understand if the message thread CPUs are actually getting used that
often for workers, or if the exclusion makes our idle CPU hunt slower
somehow.

-chris