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

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Message-ID: <20120518140945.GN20215@aftab.osrc.amd.com>
Date:	Fri, 18 May 2012 16:09:45 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <borislav.petkov@....com>
Subject: Re: [PATCH 1/2] perf: Add persistent event facilities

On Fri, May 18, 2012 at 03:37:47PM +0200, Peter Zijlstra wrote:
> On Fri, 2012-05-18 at 14:55 +0200, Borislav Petkov wrote:
> > It is an interesting question how we handle that RO/RW thing?
> 
> RO buffers overwrite, RW buffers loose events.
> 
> We could allow both for persistent under the constraints:
> 
>  VM_SHARED - must imply !VM_WRITE since multiple people writing to the
>              control page will get 'interesting' very fast.

Ok, I get that one.

>  !VM_SHARED - must fail mmap() of the debugfs file if there's already
>               an existing user.

What happens here if a second user wants to enable a second set of
persistent events, allocate a second set of per-CPU buffers?

> Note that even RW buffers only allows writes to the control page, a
> write to the actual data itself will generate a fault.

Ok.

-- 
Regards/Gruss,
Boris.

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