linux-kernel - Re: [RFC] perf: fix building for ARCv1

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075F44CAFF9@IN01WEMBXA.internal.synopsys.com>
Date:	Mon, 19 Oct 2015 09:28:43 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Aabid Rushdi <Aabid.Rushdi@...opsys.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
	"dvhart@...ux.intel.com" <dvhart@...ux.intel.com>,
	"dsahern@...il.com" <dsahern@...il.com>,
	"acme@...hat.com" <acme@...hat.com>
Subject: Re: [RFC] perf: fix building for ARCv1

On Monday 19 October 2015 11:20 AM, Andi Kleen wrote:
> Vineet Gupta <Vineet.Gupta1@...opsys.com> writes:
>> But this user space - so IMHO UP/SMP doesn't matter and we can't simulate them in
>> C just by itself.
> It matters when you access the perf ring buffer which is updated by kernel.

That's part of the problem. The issue is with atomic_* APIs proliferation in perf
user space code which assumes native atomix r-m-w support which is not always
true. So I think we still need a feature detection mechanism and if absent leave
the ball in arch court by calling arch_atomic_* which can use creative or half
working measures so perf will work to some extent atleast and not bomb outright.

Also can u please elaborate a bit on "simulate them in C" - u mean just simple
unprotected LD, OP, ST or do u fancy usage of futex etc?

> Also perf is now multi threaded to some degree.


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