Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Wed, Dec 18, 2013 at 03:23:41PM +0200, Alexander Shishkin wrote:
>> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>> 
>> > On Wed, Dec 11, 2013 at 02:36:16PM +0200, Alexander Shishkin wrote:
>> >> Instruction tracing PMUs are capable of recording a log of instruction
>> >> execution flow on a cpu core, which can be useful for profiling and crash
>> >> analysis. This patch adds itrace infrastructure for perf events and the
>> >> rest of the kernel to use.
>> >> 
>> >> Since such PMUs can produce copious amounts of trace data, it may be
>> >> impractical to process it inside the kernel in real time, but instead export
>> >> raw trace streams to userspace for subsequent analysis. Thus, itrace PMUs
>> >> may export their trace buffers, which can be mmap()ed to userspace from a
>> >> perf event fd with a PERF_EVENT_ITRACE_OFFSET offset. To that end, perf
>> >> is extended to work with multiple ring buffers per event, reusing the
>> >> ring_buffer code in an attempt to reduce complexity.
>> >
>> > Please read the thread here: https://lkml.org/lkml/2008/12/4/64
>> >
>> > On my thoughts of this creative mmap() usage.
>> 
>> That's unfortunate, it made sense to me. But let's then have a look at
>> the alternative approaches. Bearing in mind that it is crucial for us to
>> export trace buffers to userspace as opposed to processing the trace
>> data in the kernel, the fact that we still need the normal perf data
>> stream and your dislike for mmap trickery, we need two separate file
>> descriptors: one for the perf data and one for the trace data.
>
> Why don't you start by explaining _why_ you need a second stream to
> begin with?

Oh, I'm sure I've explained it earlier ([1], [2]), but why not. The data
in the second stream is generated at a rate which is hundreds of
megabytes per second per core. Decoding this data is ~1000 times slower
than generating it. Ergo, can't be done in kernel, needs to be exported
as-is to userspace for later retreival and decoding. Doing it via perf
stream means an extra copy, which at these rates is a waste. Ergo, a
second buffer.

[1] https://lkml.org/lkml/2013/12/11/213
[2] https://lkml.org/lkml/2013/12/11/358

Regards,
--
Alex
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