Re: [PATCH v4 4/4] KVM: selftests: Run dirty_log_perf_test on specific CPUs

From: Sean Christopherson <seanjc@google.com>
To: Vipin Sharma <vipinsh@google.com>
Cc: pbonzini@redhat.com, dmatlack@google.com, andrew.jones@linux.dev,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 4/4] KVM: selftests: Run dirty_log_perf_test on specific CPUs
Date: Fri, 7 Oct 2022 00:14:46 +0000	[thread overview]
Message-ID: <Yz9vdoiq+0TyaGqo@google.com> (raw)
In-Reply-To: <CAHVum0e4fiaB7hGSA6z1SaiZ1632f9Md2p0Nw6G=5wqhHYvdJg@mail.gmail.com>

On Thu, Oct 06, 2022, Vipin Sharma wrote:
> On Thu, Oct 6, 2022 at 12:50 PM Sean Christopherson <seanjc@google.com> wrote:
> > > +{
> > > +     cpu_set_t cpuset;
> > > +     int err;
> > > +
> > > +     CPU_ZERO(&cpuset);
> > > +     CPU_SET(pcpu, &cpuset);
> >
> > To save user pain:
> >
> >         r = sched_getaffinity(0, sizeof(allowed_mask), &allowed_mask);
> >         TEST_ASSERT(!r, "sched_getaffinity failed, errno = %d (%s)", errno,
> >                     strerror(errno));
> >
> >         TEST_ASSERT(CPU_ISSET(pcpu, &allowed_mask),
> >                     "Task '%d' not allowed to run on pCPU '%d'\n");
> >
> >         CPU_ZERO(&allowed_mask);
> >         CPU_SET(cpu, &allowed_mask);
> >
> > that way the user will get an explicit error message if they try to pin a vCPU/task
> > that has already been affined by something else.  And then, in theory,
> > sched_setaffinity() should never fail.
> >
> > Or you could have two cpu_set_t objects and use CPU_AND(), but that seems
> > unnecessarily complex.
> >
> 
> sched_setaffinity() doesn't fail when we assign more than one task to
> the pCPU, it allows multiple tasks to be on the same pCPU. One of the
> reasons it fails is if it is provided a cpu number which is bigger
> than what is actually available on the host.
> 
> I am not convinced that pinning vCPUs to the same pCPU should throw an
> error. We should allow if someone wants to try and compare performance
> by over subscribing or any valid combination they want to test.

Oh, I'm not talking about the user pinning multiple vCPUs to the same pCPU via
the test, I'm talking about the user, or more likely something in the users's
environment, restricting what pCPUs the user's tasks are allowed on.  E.g. if
the test is run in shell that has been restricted to CPU8 via cgroups, then
sched_setaffinity() will fail if the user tries to pin vCPUs to any other CPU.