linux-kernel - Re: [tip:perf/core] tools/perf: Add required memory barriers

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Message-ID: <20131107082122.GC32438@gmail.com>
Date:	Thu, 7 Nov 2013 09:21:22 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vince Weaver <vince@...ter.net>, hpa@...or.com, anton@...ba.org,
	mathieu.desnoyers@...ymtl.ca, linux-kernel@...r.kernel.org,
	michael@...erman.id.au, paulmck@...ux.vnet.ibm.com,
	benh@...nel.crashing.org, fweisbec@...il.com, VICTORK@...ibm.com,
	tglx@...utronix.de, oleg@...hat.com, mikey@...ling.org,
	linux-tip-commits@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>
Subject: Re: [tip:perf/core] tools/perf: Add required memory barriers

* Peter Zijlstra <peterz@...radead.org> wrote:

> > Requiring the user of a kernel interface to have a deep knowledge of 
> > optimizing compilers, barriers, and CPU memory models is just asking 
> > for trouble.
> 
> It shouldn't be all that hard to put this in a (lgpl) library others can 
> link to -- that way you can build it once (using GCC).

I'd suggest to expose it via a new perf syscall, using vsyscall methods to 
not have to enter the kernel for the pure user-space bits. It should also 
have a real usecase in tools/perf/ so that it's constantly tested, with 
matching 'perf test' entries, etc.

I don't want a library that is external and under-tested: for example 
quite a few of the PAPI breakages were found very late, after a new kernel 
has been released - that's the big disadvantage of librarization and 
decentralization. The decentralized library model might work if all you 
want to create is a second-class also-ran GUI, but it just doesn't work 
very well for actively developed kernel code.

Thanks,

	Ingo
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