Re: [PATCH] perf record: Add snapshot mode support for perf's regular events

["Wangnan (F)" <wangnan0@huawei.com>]

On 2015/11/26 17:19, Ingo Molnar wrote:
> * Wangnan (F) <wangnan0@huawei.com> wrote:
>
>>
>> On 2015/11/25 20:20, Peter Zijlstra wrote:
>>> On Wed, Nov 25, 2015 at 05:44:00PM +0800, Wangnan (F) wrote:
>>>> On 2015/11/25 17:27, Peter Zijlstra wrote:
>>>>> On Tue, Nov 24, 2015 at 10:00:31PM +0800, Yunlong Song wrote:
>>>>>> In our patch, we create and maintain a user space ring buffer to store
>>>>>> perf's tracing info, instead of directly writing to perf.data file as
>>>>>> before. In snapshot mode, only a SIGUSR2 signal can trigger perf to dump
>>>>>> the tracing info currently stored in the user space ring buffer to
>>>>>> perf.data file.
>>>>> I would very much like to first fix the perf overwrite mode: see
>>>>> lkml.kernel.org/r/20151023151205.GW11639@twins.programming.kicks-ass.net
>>>> I think they can be done in parallel. We can first do something with
>>>> tracking events and perf's output file, and wait for kernel level
>>>> overwrite mode fixed, then decide whether to implement perf's own
>>>> ringbuffer.
>>> That seems backwards; why would you ever want to endlessly copy the
>>> events if you're not going to use them?
>> I agree that we need to fixing overwrite mode. However, user space ringbuffer
>> can be more flexible. for example, dynamically shrinking and expansion. It would
>> be hard in kernel I think, and I'm not sure how much flexibility we need. Doing
>> things in kernel always more difficult than in userspace.
>>
>> But yes, we can do that userspace ring buffer when we really need it. At very
>> first we can start working on perf side and assume overwrite mode is ready.
> I don't think Peter asked for much: pick up the patch he has already written and
> use it, to have an even lower overhead always-enabled background tracing mode of
> perf.
>
> Resizing shouldn't be much of an issue with existing features: if events start
> overflowing or some other threshold for dynamic increase of the ring-buffer is met
> then the daemon should open a new set of events with a larger ring-buffer, and
> close the old events once the new tracing ring-buffer is up and running.
>
> Use event multiplexing to output all interesting events into the same single (per
> CPU) ring-buffer.

Thank you for your reply. We'll start looking Peter's patch and try
to find what should we do to make it upstream faster. Could you please
kindly give some hints?