Re: [PATCH 1/2] perf: Allow non-contiguous AUX buffer pages via PMU capability

[Yabin Cui <yabinc@google.com>]

On Tue, Apr 29, 2025 at 10:02 AM Yabin Cui <yabinc@google.com> wrote:
>
> On Mon, Apr 28, 2025 at 1:56 AM James Clark <james.clark@linaro.org> wrote:
> >
> >
> >
> > On 23/04/2025 8:52 pm, Yabin Cui wrote:
> > > On Tue, Apr 22, 2025 at 7:10 AM Leo Yan <leo.yan@arm.com> wrote:
> > >>
> > >> On Tue, Apr 22, 2025 at 02:49:54PM +0200, Ingo Molnar wrote:
> > >>
> > >> [...]
> > >>
> > >>>> Hi Yabin,
> > >>>>
> > >>>> I was wondering if this is just the opposite of
> > >>>> PERF_PMU_CAP_AUX_NO_SG, and that order 0 should be used by default
> > >>>> for all devices to solve the issue you describe. Because we already
> > >>>> have PERF_PMU_CAP_AUX_NO_SG for devices that need contiguous pages.
> > >>>> Then I found commit 5768402fd9c6 ("perf/ring_buffer: Use high order
> > >>>> allocations for AUX buffers optimistically") that explains that the
> > >>>> current allocation strategy is an optimization.
> > >>>>
> > >>>> Your change seems to decide that for certain devices we want to
> > >>>> optimize for fragmentation rather than performance. If these are
> > >>>> rarely used features specifically when looking at performance should
> > >>>> we not continue to optimize for performance? Or at least make it user
> > >>>> configurable?
> > >>>
> > >>> So there seems to be 3 categories:
> > >>>
> > >>>   - 1) Must have physically contiguous AUX buffers, it's a hardware ABI.
> > >>>        (PERF_PMU_CAP_AUX_NO_SG for Intel BTS and PT.)
> > >>>
> > >>>   - 2) Would be nice to have continguous AUX buffers, for a bit more
> > >>>        performance.
> > >>>
> > >>>   - 3) Doesn't really care.
> > >>>
> > >>> So we do have #1, and it appears Yabin's usecase is #3?
> > >
> > > Yes, in my usecase, I care much more about MM-friendly than a little potential
> > > performance when using PMU. It's not a rarely used feature. On Android, we
> > > collect ETM data periodically on internal user devices for AutoFDO optimization
> > > (for both userspace libraries and the kernel). Allocating a large
> > > chunk of contiguous
> > > AUX pages (4M for each CPU) periodically is almost unbearable. The kernel may
> > > need to kill many processes to fulfill the request. It affects user
> > > experience even
> > > after using PMU.
> > >
> > > I am totally fine to reuse PERF_PMU_CAP_AUX_NO_SG. If PMUs don't want to
> > > sacrifice performance for MM-friendly, why support scatter gather mode? If there
> > > are strong performance reasons to allocate contiguous AUX pages in
> > > scatter gather
> > > mode, I hope max_order is configurable in userspace.
> > >
> > > Currently, max_order is affected by aux_watermark. But aux_watermark
> > > also affects
> > > how frequently the PMU overflows AUX buffer and notifies userspace.
> > > It's not ideal
> > > to set aux_watermark to 1 page size. So if we want to make max_order user
> > > configurable, maybe we can add a one bit field in perf_event_attr?
> > >
> > >>
> > >> In Yabin's case, the AUX buffer work as a bounce buffer.  The hardware
> > >> trace data is copied by a driver from low level's contiguous buffer to
> > >> the AUX buffer.
> > >>
> > >> In this case we cannot benefit much from continguous AUX buffers.
> > >>
> > >> Thanks,
> > >> Leo
> >
> > Hi Yabin,
> >
> > So after doing some testing it looks like there is 0 difference in
> > overhead for max_order=0 vs ensuring the buffer is one contiguous
> > allocation for Arm SPE, and TRBE would be exactly the same. This makes
> > sense because we're vmapping pages individually anyway regardless of the
> > base allocation.
> >
> > Seems like the performance optimization of the optimistically large
> > mappings is only for devices that require extra buffer management stuff
> > other than normal virtual memory. Can we add a new capability
> > PERF_PMU_CAP_AUX_PREFER_LARGE and apply it to Intel PT and BTS? Then the
> > old (before the optimistic large allocs change) max_order=0 behavior
> > becomes the default again, and PREFER_LARGE is just for those two
> > devices. Other and new devices would get the more memory friendly
> > allocations by default, as it's unlikely they'll benefit from anything
> > different.
> >
> Good suggestion! I will upload a v2 patch for that.

Hi everyone,
I have sent the v2 patch for review, with the title
"[PATCH v2] perf: Allocate non-contiguous AUX pages by default".
Please help review it. Thanks!

> >
> > Thanks
> > James
> >