Re: perf: is it possible to userspace rdpmc but only on a certain core type

[Mathieu Desnoyers]

On 2025-01-21 07:52, Peter Zijlstra wrote:
> On Mon, Jan 20, 2025 at 11:44:37AM -0500, Liang, Kan wrote:
> > On 2025-01-17 5:04 p.m., Vince Weaver wrote:
> > > Hello
> > >
> > > so we've been working on PAPI support for Intel Top-Down events, which
> > > let's say does "exciting" things involving the rdpmc instruction.
> > >
> > > One issue we are having is that on a hybrid machine (Raptor Lake in this
> > > case with performance/efficiency cores) there is no top-down support
> > > for the E-cores, and it will gpf/segfault if you try to rdpmc the top-down
> > > events.
> > >
> > > Obviously PAPI would like to avoid this, and somehow only run the rdpmc
> > > from userspace if scheduled on a P-core.
> > >
> > > Is there any way to atomically do this?  Somehow detect what core we are
> > > on and atomically execute a userspace instruction before a core-reschedule
> > > can happen?
> > >
> > > Or barring that, any other way to handle this in a way that won't crash
> > > without having to have the users have to bind to a core any time they want
> > > to run PAPI?
> >
> > Can the PAPI rely on the event_idx(), similar to what Andi's pmu-tools
> > do? For a stopped event, the index is always 0.
>
> That's not race-free, the task can get migrated to an E core the moment
> after you done the load and before the rdpmc instruction.
>
> I suppose you can wrap the whole thing in RSEQ though, it's a bit of a
> pain, but RSEQ can be configured to abort on migration.
>
> The very latest libc (2.35+) should have rseq registered by default,
> older will have to do so itself -- there is example code in
> tools/testing/selftests/rseq but also
> https://git.kernel.org/pub/scm/libs/librseq/librseq.git

Indeed, you could start from a copy of this function:

https://git.kernel.org/pub/scm/libs/librseq/librseq.git/tree/include/rseq/arch/x86/bits.h#n161

and tweak it to issue "rdpmc" rather than "addq", thus creating a helper
such as:

int rseq_try_rdpmc(params..., int cpu);
(e.g. return 0 on success, -1 on abort)

and use it as such from C (untested code snippet):

static inline bool rseq_rdpmc(params...)
{
    bool rdpmc_issued = false;

    for (;;) {
        int cpu = rseq_current_cpu();

        if (!cpu_is_p_core(cpu))
           break;
        if (!rseq_try_rdpmc(params..., cpu)) {
           rdpmc_issued = true;
           break;
        }
    }
    return rdpmc_issued;
}

The rseq critical section in rseq_try_rdpmc will either abort if migrated
elsewhere, else it will issue the rdpmc instruction if it is still on the
right cpu when the instruction is executed.

Thanks,

Mathieu

>

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com