RE: [PATCH 0/4] riscv: Allow userspace to directly access perf counters

[David Laight]

From: Atish Patra
> Sent: 13 April 2023 20:18
> 
> On Thu, Apr 13, 2023 at 9:47 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
> >
> > riscv used to allow direct access to cycle/time/instret counters,
> > bypassing the perf framework, this patchset intends to allow the user to
> > mmap any counter when accessed through perf. But we can't break the
> > existing behaviour so we introduce a sysctl perf_user_access like arm64
> > does, which defaults to the legacy mode described above.
> >
> 
> It would be good provide additional direction for user space packages:
> 
> The legacy behavior is supported for now in order to avoid breaking
> existing software.
> However, reading counters directly without perf interaction may
> provide incorrect values which
> the userspace software must avoid. We are hoping that the user space
> packages which
> read the cycle/instret directly, will move to the proper interface
> eventually if they actually need it.
> Most of the users are supposed to read "time" instead of "cycle" if
> they intend to read timestamps.

If you are trying to measure the performance of short code
fragments then you need pretty much raw access directly to
the cycle/clock count register.

I've done this on x86 to compare the actual cycle times
of different implementations of the IP checksum loop
(and compare them to the theoretical limit).
The perf framework just added far too much latency,
only directly reading the cpu registers gave anything
like reliable (and consistent) answers.

Clearly process switches (especially cpu migrations) cause
problems, but they are obviously invalid values and can
be ignored.

So while a lot of uses may be 'happy' with the values the
perf framework gives, sometimes you do need to directly
read the relevant registers.

	David

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