linux-kernel - Re: [PATCH v5 4/4]: perf/core: complete replace of lists by rb trees for pinned and flexible groups at perf_event

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Message-ID: <87h8yavv20.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Tue, 18 Jul 2017 14:33:27 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>, Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/4]: perf/core: complete replace of lists by rb trees for pinned and flexible groups at perf_event_context

Alexey Budankov <alexey.budankov@...ux.intel.com> writes:

> Hi,

Hi,

> Are there any new comments so far? Could you please suggest further steps forward?

Apparently the patches are not threaded, so one needs to fish them out
one by one in order to review.

> On 10.07.2017 16:03, Alexey Budankov wrote:
>> perf/core: complete replace of lists by rb trees for pinned and 
>>            flexible groups at perf_event_context

No need to duplicate the subject line here. Also, it can be more concise
than this like "perf: Replace context's pinned/flexible lists with trees".

>> By default, the userspace perf tool opens per-cpu task-bound events
>> when sampling, so for N logical events requested by the user, the tool
>> will open N * NR_CPUS events.
>> 
>> In the kernel, we mux events with a hrtimer, periodically rotating the
>> flexible group list and trying to schedule each group in turn. We skip
>> groups whose cpu filter doesn't match. So when we get unlucky, we can
>> walk N * (NR_CPUS - 1) groups pointlessly for each hrtimer invocation.
>> 
>> This has been observed to result in significant overhead when running
>> the STREAM benchmark on 272 core Xeon Phi systems.
>> 
>> One way to avoid this is to place our events into an rb tree sorted by
>> CPU filter, so that our hrtimer can skip to the current CPU's
>> list and ignore everything else.

It looks like these 4 paragraphs are repeated in every patch.

>> This patch implements complete replacement of lists by rb trees for 
>> pinned and flexible groups.

And this is the actually informative part.

>> The patch set was tested on Xeon Phi using perf_fuzzer and tests 
>> from here: https://github.com/deater/perf_event_tests

Although this is also useful.

>> The full patch set (v1-4) is attached for convenience. 
>> 
>> Branch revision:
>> * perf/core 007b811b4041989ec2dc91b9614aa2c41332723e 
>>   Merge tag 'perf-core-for-mingo-4.13-20170719' of 
>>   git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux into perf/core

Not sure what this is, though.

As has been recently pointed out elsewhere, you can get a good idea of
how to structure and format commit messages for a particular piece of
code by looking at 'git log path/to/code' and paying attention to common
patterns.

Thanks,
--
Alex