Re: [PATCH 4/5] perf record: mmap output file - v5

[David Ahern <dsahern@gmail.com>]

On 11/19/13, 8:31 AM, Ingo Molnar wrote:
>> The only reason I reacted was because the changelog mentioned
>> avoiding a feedback loop -- so I obviously had to point out that it
>> didn't do such a thing, it only changed the details of the loop.
>
> So with MAP_POPULATE the 'feedback window' is moved entirely into the
> kernel (to within a single syscall) and is also reduced significantly,
> compared to a write() loop.

As I understand it we have to use MAP_SHARED, not MAP_PRIVATE for files. 
So MAP_POPULATE does not work here. (And I tried to verify -- 
MAP_PRIVATE | MAP_POPULATE drops the feedback loop, but the file is 0's 
after the header).

>> I'm fairly certain this particular problem is unavoidable, no matter
>> what the mechanism used, you can always create feedback.
>
> Well, we could exclude the profiling task itself from profiling events
> (just like ftrace and core bits of perf does it out of necessity), but
> I intentionally wanted to avoid that, to make sure we are honest and
> to make sure people don't tolerate profiling overhead that disturbs
> other workloads.

Samples generated by perf itself need to be observable -- e.g. process 
scheduling I want to see the time consumed by the data collector itself 
and there are times when 'perf trace -- perf ...' is useful.

perf just needs options to do the right thing and stay out of its own 
way. Having a restriction that you can't do system wide collection of 
systems calls AND faults does not seem all that limiting.

David