Re: [RFC 2/2] perf: add AUX area to ring buffer for raw data streams

[Alexander Shishkin]

Robert Richter <rric@xxxxxxxxxx> writes:

> On 15.05.14 18:08:30, Alexander Shishkin wrote:
>> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> 
>> This patch introduces "AUX space" in the perf mmap buffer, intended for
>> exporting high bandwidth data streams to userspace, such as instruction
>> flow traces.
>> 
>> AUX space is a ring buffer, defined by aux_{offset,size} fields in the
>> user_page structure, and read/write pointers aux_{head,tail}, which abide
>> by the same rules as data_* counterparts of the main perf buffer.
>> 
>> In order to allocate/mmap AUX, userspace needs to set up aux_offset to
>> such an offset that will be greater than data_offset+data_size and
>> aux_size to be the desired buffer size. Both need to be page aligned.
>> The latter is not forced to be a power of 2 number of pages, so that PMU
>> drivers have to take care of this if necessary in their implementations
>> of ->alloc_aux(). Then, same aux_offset and aux_size should be passed to
>> mmap() call and if everything adds up, you should have an AUX buffer as
>> a result.
>
> Raw data sample (PERF_SAMPLE_RAW) already allow to pass data to
> userspace. What is the reason for a separate buffer? I see the
> following:
>
>  * the buffer accessed from userspace should have a customized format
>    unrelated to perf,
>
>  * zero copying (not sure if this is really possible),
>
>  * data size and alignment requirements that can not be handled by raw
>    data samples.

It is all three. What we're dealing with at the moment is a unit that
generates instruction traces at rates like hhundreds of megabytes per
second. So the only sensible thing to do is zero copy the data.

> If possible, raw data samples should be a first option.
>
> On the other side there might the need of an aux buffer (I would
> better call it raw buffer analog to raw data). So here some comments
> on the implementation and the user i/f.
>
> Instead of extending the current ringbuffer I would better add a 2nd
> ring buffer. The mmap and ringbuffer handling and esp. the access from
> userspace looks much more complex now and it was already complex
> before. So with a 2nd buffer there would be less changes in the
> implementation and handling. Access from userland would remain as it
> currently is, there is no need to modify the header page layout. To
> setup a 2nd buffer, you just need to modify the mmap call in a way
> (maybe by prot, flags or offset argument) to let the kernel know an
> aux buffer should be attached to the event.

Tried that, didn't fly. See the full story here [1]. Though splicing
arbitrary pages into perf buffer is less than trivial, especially so
with hardware that doesn't do scatter-gather. So all in all, AUX area
seems to be a sensible compromise.

[1] http://marc.info/?l=linux-kernel&m=139264764530809&w=2

Regards,
--
Alex
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