Re: [PATCH 3/7] perf: arm64: Use rseq to test userspace access to pmu counters

[Raphael Gault <raphael.gault@arm.com>]

Hi Mathieu, Mark,

On 6/11/19 8:33 PM, Mathieu Desnoyers wrote:
> ----- On Jun 11, 2019, at 6:57 PM, Mark Rutland mark.rutland@arm.com wrote:
> 
>> Hi Arnaldo,
>>
>> On Tue, Jun 11, 2019 at 11:33:46AM -0300, Arnaldo Carvalho de Melo wrote:
>>> Em Tue, Jun 11, 2019 at 01:53:11PM +0100, Raphael Gault escreveu:
>>>> Add an extra test to check userspace access to pmu hardware counters.
>>>> This test doesn't rely on the seqlock as a synchronisation mechanism but
>>>> instead uses the restartable sequences to make sure that the thread is
>>>> not interrupted when reading the index of the counter and the associated
>>>> pmu register.
>>>>
>>>> In addition to reading the pmu counters, this test is run several time
>>>> in order to measure the ratio of failures:
>>>> I ran this test on the Juno development platform, which is big.LITTLE
>>>> with 4 Cortex A53 and 2 Cortex A57. The results vary quite a lot
>>>> (running it with 100 tests is not so long and I did it several times).
>>>> I ran it once with 10000 iterations:
>>>> `runs: 10000, abort: 62.53%, zero: 34.93%, success: 2.54%`
>>>>
>>>> Signed-off-by: Raphael Gault <raphael.gault@arm.com>
>>>> ---
>>>>   tools/perf/arch/arm64/include/arch-tests.h    |   5 +-
>>>>   tools/perf/arch/arm64/include/rseq-arm64.h    | 220 ++++++++++++++++++
>>>
>>> So, I applied the first patch in this series, but could you please break
>>> this patch into at least two, one introducing the facility
>>> (include/rseq*) and the second adding the test?
>>>
>>> We try to enforce this kind of granularity as down the line we may want
>>> to revert one part while the other already has other uses and thus
>>> wouldn't allow a straight revert.
>>>
>>> Also, can this go to tools/arch/ instead? Is this really perf specific?
>>> Isn't there any arch/arm64/include files for the kernel that we could
>>> mirror and have it checked for drift in tools/perf/check-headers.sh?
>>
>> The rseq bits aren't strictly perf specific, and I think the existing
>> bits under tools/testing/selftests/rseq/ could be factored out to common
>> locations under tools/include/ and tools/arch/*/include/.
> 
> Hi Mark,
> 
> Thanks for CCing me!
> 
> Or into a stand-alone librseq project:
> 
> https://github.com/compudj/librseq (currently a development branch in
> my own github)
> 
> I don't see why this user-space code should sit in the kernel tree.
> It is not tooling-specific.
> 

I understand your point but I have to admit that I don't really see how 
to make it work together with the test which require those definitions.

>>
>>  From a scan, those already duplicate barriers and other helpers which
>> already have definitions under tools/, which seems unfortunate. :/
>>

Also I realize that there is a duplicate with definitions introduced in 
the selftests but I kind of simplified the macros I'm using to get rid 
of what wasn't useful to me at the moment. (mainly the loop labels and 
parameter injections in the asm statement)
I understand what both Mark and Arnaldo are saying about moving it out 
of perf so that it is not duplicated but my question is whether it is a 
good thing to do as is since it is not exactly the same content as 
what's in the selftests.

I hope you can understand my concerns and I'd like to hear your opinions 
on that matter.

Thanks,

-- 
Raphael Gault

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