Re: [PATCH 1/2] perf: Add persistent event facilities

[Borislav Petkov]

On Fri, May 18, 2012 at 01:59:57PM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Fri, 2012-05-18 at 13:03 +0200, Borislav Petkov wrote:
> > > >  - you allow an arbitrary place in debugfs; this might make finding
> > > > them 'interesting'. Should we put them all in the same place?
> > > 
> > > My take on this is that we want to be able to make the same events we
> > > have now, persistent. Basically not trace for the duration of a child
> > > process but in a process-agnostic way, system-wide.
> > 
> > This would argue against per-task persistent events.. 
> 
> Yeah, 'persistency' is something that's per CPU at minimum.

Yeah, forget the per-task thing - that's me not understanding the code
fully. They need to be per CPU and "taskless."

> > Note the plural there, it might be very desirable to allow 
> > multiple events into a single persistent buffer.
> 
> very, very, very much so. One (per CPU) buffer, with many events 
> multiplexed into it.

Which begs the other question: mce could use buffers which are RO - at
least, I don't see a usecase where we want to delete entries from them -
but other persistent events users might want to delete those events to
free up the buffers.

It is an interesting question how we handle that RO/RW thing? Maybe per
event and do something of a kernel-side filtering which Peter didn't
like in Robert's IBS stuff.

Hmmm...

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