linux-kernel - Re: [PATCH v6 5/5] KVM: selftests: Allowing running dirty_log_perf

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Date:   Thu, 27 Oct 2022 13:02:49 -0700
From:   Vipin Sharma <vipinsh@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Wang, Wei W" <wei.w.wang@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "andrew.jones@...ux.dev" <andrew.jones@...ux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 5/5] KVM: selftests: Allowing running
 dirty_log_perf_test on specific CPUs

On Thu, Oct 27, 2022 at 8:56 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Oct 27, 2022, Wang, Wei W wrote:
> > On Wednesday, October 26, 2022 11:44 PM, Sean Christopherson wrote:
> > > If we go this route in the future, we'd need to add a worker trampoline as the
> > > pinning needs to happen in the worker task itself to guarantee that the pinning
> > > takes effect before the worker does anything useful.  That should be very
> > > doable.
> >
> > The alternative way is the one I shared before, using this:
> >
> > /* Thread created with attribute ATTR will be limited to run only on
> >    the processors represented in CPUSET.  */
> > extern int pthread_attr_setaffinity_np (pthread_attr_t *__attr,
> >                                  size_t __cpusetsize,
> >                                  const cpu_set_t *__cpuset)
> >
> > Basically, the thread is created on the pCPU as user specified.
> > I think this is better than "creating the thread on an arbitrary pCPU
> > and then pinning it to the user specified pCPU in the thread's start routine".
>
> Ah, yeah, that's better.
>

pthread_create() will internally call sched_setaffinity() syscall
after creation of a thread on a random CPU. So, from the performance
side there is not much difference between the two approaches.

However, we will still need pin_this_task_to_pcpu()/sched_affinity()
to move the main thread to a specific pCPU, therefore, I am thinking
of keeping the current approach unless there is a strong objection to
it.

> > Probably we also don't need "bool pin_vcpus".
>
> Yeah, but for selftests shaving bytes is not exactly top priority, and having a
> dedicated flag avoids the need for magic numbers.  If Vipin had used -1, I'd
> probably be fine with that, but I'm also totally fine using a dedicated flag too.
>

Same, it is not performance critical in this case to add a magical -1.