linux-kernel - Re: [PATCH 0/8] perf: add ability to sample physical data addresses

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Message-ID: <20130628095828.GG29209@dyad.programming.kicks-ass.net>
Date:	Fri, 28 Jun 2013 11:58:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung.kim@....com>
Subject: Re: [PATCH 0/8] perf: add ability to sample physical data addresses

On Wed, Jun 26, 2013 at 09:10:50PM +0200, Stephane Eranian wrote:
> After more investigation with the author of the false sharing
> detection tool, I think
> that if the mapping changes, it is okay. The tool can detect this and
> drop the analysis
> at that address. So as long as we can flag the mapping change, we are
> okay. Hopefully,
> it does not occur frequently. If so, then I think there are bigger
> issues to fix on the system
> than false sharing.

But if you index everything using dev:inode:offset it doesn't matter if the
mapping changes; you're invariant to map placement.

And the thing is; we already compute most of that anyway in order to find the
code in DSOs, except there we use filename instead of dev:inode. However if
there were means to open files using dev:inode that might actually be more
reliable than using the filename.
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