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

[Wangnan (F)]

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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > 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.

Thank you.

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