Re: Per thread sampling

[Manuel Selva <selva.manuel@gmail.com>]

Thanks for the answer Jirka. I saw this parameter and was wondering what 
was its purpose (because the name is
perf_event_mlock_kb and not
perf_event_mmap_kb).

Manu


On 02/20/2014 09:04 AM, Jiri Olsa wrote:
> On Wed, Feb 19, 2014 at 04:00:45PM +0100, Manuel Selva wrote:
>> Hi all,
>>
>> I just tried in a 2-threaded application to call twice (one call for
>> each thread) perf_event_open syscall with parameters configured to do
>> sampling on MEM INST RETIRED.LATENCY ABOVE THRESHOLD event. Both call
>> succeed.
>>
>> "mmaping" the file descriptor return by the first call succeed but
>> "mmaping" the second file descriptor results in a "Operation not
>> permitted" error (errno = 1).
>
> seems like you might have crossed the perf mem user limit?
>
> [jolsa@krava ~]$ sysctl kernel.perf_event_mlock_kb
> kernel.perf_event_mlock_kb = 516
>
> you can either split the memory amount between those
> threads or you can increase that limit.. or run your
> app under root ;-)
>
> jirka
>
>> mit
>> A work around could be to sample all threads (with pid = -1) including
>> pid and tid in samples and filter samples at processing time. Before
>> switching to this solution I wanted to ask if this a known limitation
>> of the syscall, an error from my side, or a bug  (the man page doesn't
>> answer to this question) ?
>>
>> Thanks,
>>
>> Manu
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>