Re: [PATCH 2/2] x86/resctrl: Don't workqueue local event counter reads

[Fenghua Yu]

Hi, Tony,

On 11/4/24 16:12, Luck, Tony wrote:
> > Whenever this function is called, the performance is degraded rather
> > than improved because extra get_cpu()/put_cpu() are called in the fast
> > path in the current patch.
>
> But get_cpu()/put_cpu() aren't high overhead. Maybe costs less that the
> cpumask_any_housekeeping() call that is avoided by Peter's patch.

Quote from Peter:

"AMD EPYC 7B12 64-Core Processor (250 mon groups)

Local Domain:   3.25M -> 1.22M (-62.5%)
Remote Domain:  7.91M -> 8.05M (+2.9%)

Intel(R) Xeon(R) Gold 6268CL CPU @ 2.80GHz (190 mon groups)

Local Domain:   2.98M -> 2.21M (-25.8%)
Remote Domain:  4.49M -> 4.62M (+3.1%)

Note that there is a small increase in overhead for remote domains,
which results from the introduction of a put_cpu() call to reenable
preemption after determining whether the fast path can be used."

As his data shows, if the fast path is not taken, the extra put_cpu() 
itself costs +2.9% extra time on AMD machine and +3.1% extra time on 
Intel machine.

And this ~3% overhead is on top of queued work, which is more expensive 
than cpumask_any_housekeeping() IIUC.

> Note that if Peter's patch doesn't take its fast path because the calling
> CPU was on the wrong domain, then the subsequent code is going to
> do an IPI whichever of the if/else path is taken.

In this case, actually IPI is only taken in smp_call_function_any() and 
smp_call_on_cpu() invokes a queued work instead of IPI.

My proposed change logically doesn't change Peter's fast path and 
performance for nohz_full/smp_call_on_cpu() case. It just utilizes the 
"built-in fast path already" inside smp_call_function_any() to save 
extra get_cpu() and put_cpu(). Hopefully the saved extra get_cpu() and 
put_cpu() can offset cost of cpumask_any_housekeeping().

From Peter's commit message, seems nohz_full case is not 
called/measured a lot if any. If only one or a very few housekeeping 
CPUs on a large system, the nohz_full case will be called frequently and 
fast path will fail most of time and the extra get_cpu()/put_cpu() 
around the fast path might impact more on both local and total domain.

Thanks.

-Fenghua