linux-kernel - Re: [PATCH v1 0/3] perf: expose thread context switch out event type to user space

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Date:   Mon, 5 Mar 2018 19:20:00 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>
Subject: Re: [PATCH v1 0/3] perf: expose thread context switch out event type
 to user space

Hi Arnaldo,

On 05.03.2018 18:06, Arnaldo Carvalho de Melo wrote:
> Em Mon, Mar 05, 2018 at 02:35:02PM +0300, Alexey Budankov escreveu:
>>
>> Here is a series of small patches that implement exposing type of 
>> context-switch-out event as a part of PERF_RECORD_SWITCH[_CPU_WIDE] record.
>>
>> Introduced types of context-switch-out events assumed to be:
>> a) preempt: task->state == TASK_RUNNING
>> 	misc &= PERF_RECORD_MISC_SWITCH_OUT
>> 	
>> b) yield: !preempt - using new bit PERF_RECORD_MISC_SWITCH_OUT_YIELD:
>  	misc &= PERF_RECORD_MISC_SWITCH_OUT|PERF_RECORD_MISC_SWITCH_OUT_YIELD
>>
>> Perf tool report and script commands output has been extended to decode 
>> new yield bit and the updated output looks like in the examples below.
> 
> I'm just waiting for the current reviewers to be satisfied with this,
> but I think this is a great addition and 'perf trace' is another tool
> that should jump into this, showing forced context switches together
> with syscalls.

It's great to know there is a value in that change for other Perf tools.

Extending perf trace (strace inspired) tool in that respect might makes 
sense. I anticipate possible tracing overhead needs to be expected and 
probably handled somehow.

But, anyway, yep, per-thread syscall traces enriched with typed context 
switch boundaries may be the great extension, in comparison to the 
original strace tool.

IMHO, some simple summary metrics like amount of preempt or yield 
context switches (per-thread or per-process) could bring even more 
value into perf trace tool functionality.

BR,
Alexey

> 
> - Arnaldo
>  
>> The documentation has been updated to mention yield switch out events and its 
>> decoding symbols in perf script output.
>>
>> The changes have been manually tested on Fedora 27 with the patched kernel:
>> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git perf/core
>>
>> perf report -D -i system-wide.perf: 
>>
>> 0x1b9c50 [0x30]: event: 15
>> .
>> . ... raw event: size 48 bytes
>> .  0000:  0f 00 00 00 00 20 30 00 01 1e 00 00 01 1e 00 00  ..... 0.........
>> .  0010:  00 00 00 00 00 00 00 00 85 ae d4 e3 3e 0e 00 00  ............>...
>> .  0020:  54 00 00 00 00 00 00 00 05 00 00 00 00 00 00 00  T...............
>>
>> 5 15663273127557 0x1b9c50 [0x30]: PERF_RECORD_SWITCH_CPU_WIDE OUT  next pid/tid:  7681/7681 
>>
>> 0x2646c0 [0x30]: event: 15
>> .
>> . ... raw event: size 48 bytes
>> .  0000:  0f 00 00 00 00 60 30 00 00 00 00 00 00 00 00 00  .....`0.........
>> .  0010:  00 1e 00 00 00 1e 00 00 29 1e d5 e3 3e 0e 00 00  ........)...>...
>> .  0020:  56 00 00 00 00 00 00 00 07 00 00 00 00 00 00 00  V...............
>>
>> 7 15663273156137 0x2646c0 [0x30]: PERF_RECORD_SWITCH_CPU_WIDE OUT yield  next pid/tid:     0/0
>>
>> perf script --show-switch-events -F +misc -I -i system-wide.perf:
>>
>> amplxe-perf  7681 [005] S     15663.273151: PERF_RECORD_SWITCH_CPU_WIDE OUT  next pid/tid:    39/39   
>> migration/5    39 [005]       15663.273152: PERF_RECORD_SWITCH_CPU_WIDE IN   prev pid/tid:  7681/7681 
>> amplxe-perf  7680 [007] K     15663.273153:          1                                                                                                                                       context-switch: 
>>                   aaa488 schedule ([kernel.kallsyms])
>>                   1a9f50 __poll_nocancel (inlined)
>>
>> amplxe-perf  7680 [007] Sy    15663.273156: PERF_RECORD_SWITCH_CPU_WIDE OUT yield  next pid/tid:     0/0    
>> migration/5    39 [005] K     15663.273157:
>>
>> ---
>>  Alexey Budankov (3):
>> 	perf/core: store context switch out type into Perf trace
>> 	perf report: extend raw dump (-D) out with switch out event type
>> 	perf script: extend misc field decoding with switch out event type
>>   
>>  include/uapi/linux/perf_event.h          |  5 +++++
>>  kernel/events/core.c                     |  4 +++-
>>  tools/include/uapi/linux/perf_event.h    |  5 +++++
>>  tools/perf/Documentation/perf-script.txt | 17 +++++++++--------
>>  tools/perf/builtin-script.c              |  5 ++++-
>>  tools/perf/util/event.c                  |  4 +++-
>>  6 files changed, 29 insertions(+), 11 deletions(-)
>