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From: Namhyung Kim <namhyung@kernel.org>
To: Robert Richter <rric@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Jiri Olsa <jolsa@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 00/14] perf, persistent: Kernel updates for perf tool integration
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Date: Thu, 27 Jun 2013 14:46:36 +0900
In-Reply-To: <20130626124424.GD21788@rric.localhost> (Robert Richter's message
	of "Wed, 26 Jun 2013 14:44:24 +0200")
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Hi Robert,

On Wed, 26 Jun 2013 14:44:24 +0200, Robert Richter wrote:
> On 26.06.13 13:45:38, Ingo Molnar wrote:
>> [ How about to additional logic: attr.persistent=1 && attr.config==0 means 
>>   a new persistent event is created straight away - no ioctl is needed to 
>>   detach it explicitly. ]
>
> That's correct. We could also do the following:
>
> To connect to an existing event:
>
>  attr.type=<persistent-pmu> && attr.config==<event-id>
>
> (This might be harder to implement except the persistent event pmu
> type will be fix, PERF_TYPE_PERSISTENT=6.)
>
> To create a new persistent event:
>
>  attr.persistent=1 && attr=<some event setup: pmu, config, flags, etc>

How about using 2 bits for perfsistent flag, 1 for connecting to an
existing one, 2 for creating new one.


>> >  * The persistent event is opened with the syscall, the process gets a
>> >    new fd of the event.
>> >  * The process attaches to the event buffer with mmap.
>> 
>> Yes. And gets the pre-existing event and mmap buffer.
>
> That's what I mean.
>
> A problem here is that mmap'ed buffer size (number of pages) must be
> be equal to the pre-existing buffer size and thus to be known somehow.

What about also exporting the buffer size via sysfs pmu directory?

Thanks,
Namhyung
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