Re: [RFC PATCH] perf: Add PERF_RECORD_SWITCH to indicate context switches

[David Ahern <dsahern@gmail.com>]

On 6/12/15 4:34 AM, Adrian Hunter wrote:
> On 12/06/15 03:47, David Ahern wrote:
>> On 6/11/15 8:15 AM, Peter Zijlstra wrote:
>>>> This new PERF_RECORD_SWITCH event does not have those problems
>>>> and it also has a couple of other small advantages. It is
>>>> easier to use because it is an auxiliary event (like mmap,
>>>> comm and task events) which can be enabled by setting a single
>>>> bit. It is smaller than sched:sched_switch and easier to parse.
>>>
>>> Right, so the one wee problem I have is that this only provides sched_in
>>> data, I imagine people might be interested in sched_out as well.
>>
>> Yes and with the option of collecting callchains for sched_out.
>
> So what do you want that is different different from
> PERF_COUNT_SW_CONTEXT_SWITCHES? And why?
>

My 'perf sched timehist' command can use either sched_switch and context 
switches. As you have pointed out you need to collect them system wide 
even if you only care about a subset of tasks. If you are going to add a 
sched_in there is good symmetry by also having a sched_out that is used 
and generated in a consistent manner.