Re: [RFC 2/3] perf/x86: Control RDPMC access from .enable() hook

[Vince Weaver <vincent.weaver@maine.edu>]

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On Fri, 27 Aug 2021, Andy Lutomirski wrote:

> On Thu, Aug 26, 2021, at 12:09 PM, Rob Herring wrote:

> > After testing some scenarios and finding perf_event_tests[1], this
> > series isn't going to work for x86 unless rdpmc is restricted to task
> > events only or allowed to segfault on CPU events when read on the
> > wrong CPU rather than just returning garbage. It's been discussed
> > before here[2].
> > 
> > Ultimately, I'm just trying to define the behavior for arm64 where we
> > don't have an existing ABI to maintain and don't have to recreate the
> > mistakes of x86 rdpmc ABI. Tying the access to mmap is messy. As we
> > explicitly request user access on perf_event_open(), I think it may be
> > better to just enable access when the event's context is active and
> > ignore mmap(). Maybe you have an opinion there since you added the
> > mmap() part?
> 
> That makes sense to me. The mmap() part was always a giant kludge.
> 
> There is fundamentally a race, at least if rseq isn’t used: if you check 
> that you’re on the right CPU, do RDPMC, and throw out the result if you 
> were on the wrong CPU (determined by looking at the mmap), you still 
> would very much prefer not to fault.
> 
> Maybe rseq or a vDSO helper is the right solution for ARM.

as the author of those perf_event tests for rdpmc, I have to say if ARM 
comes up with a cleaner implementation I'd be glad to have x86 transition 
to something better.

The rdpmc code is a huge mess and has all kinds of corner cases.  I'm not 
sure anyone besides the PAPI library tries to use it, and while it's a 
nice performance improvement to use rdpmc it is really hard to get things 
working right.

As a PAPI developer we actually have run into the issue where the CPU 
switches and we were reporting the wrong results.  Also if I recall (it's 
been a while) we were having issues where the setup lets you attach to a 
process on another CPU for monitoring using the rdpmc interface and it 
returns results even though I think that will rarely ever work in 
practice.

Vince