linux-kernel - Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit

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Message-ID: <20110325192145.GA22960@elte.hu>
Date:	Fri, 25 Mar 2011 20:21:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Jack Steiner <steiner@....com>,
	Jan Beulich <JBeulich@...ell.com>,
	Borislav Petkov <bp@...64.org>,
	Nick Piggin <npiggin@...nel.dk>,
	"x86@...nel.org" <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, tee@....com,
	Nikanth Karthikesan <knikanth@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit_lock
 if possible

* Andi Kleen <andi@...stfloor.org> wrote:

> > Also seriously complicated by the kexec case where the previous kernel
> > didn't clean up PMU state. There is simply no sane way to detect if its
> 
> That's a good point, but we can easily stop the PMU before kexec.

Wrong - there's lots of existing Linux versions out there that will kexec with 
PMU state active. We cannot change them retroactively.

> > actually used and by whoem.
> 
> You check if the counter is enabled. If it's already enabled it's used by 
> someone else.

Wrong - or it can be enabled if it was left enabled accidentally. We treat PMU 
state like we treat other CPU state.

> > The whole PMU 'sharing' concept championed by Andi is utter crap.
> 
> Why? It's the same thing as having some less counters.

Wrong again - 25% or 50% of the events stolen with no user choice is a pretty 
big deal.

Consider the example in this thread: cachemiss profiling done via perf, which 
needs two events. If one of the generic counters is 'stolen' by a BIOS and 
Linux accepts this silently then it means the loss of real functionality.

In other words, '25% of a specific hardware functionality stolen by the BIOS' 
(or more) is absolutely not acceptable.

> [...] You need to already support that for architectural perfmon with 
> variable counters anyways or for sharing with oprofile.

Wrong, that's different - multiplexing the PMU is obviously done within the OS. 
It's evidently user configurable - users can use oprofile or perf - and the 
user can still fully utilise the PMU to the extent he wishes to - it's his 
hardware.

It is not possible for the kernel to stop the BIOS from using the PMU though, 
so it's not 'sharing' no matter what 'protocol' is used, it's 'stealing'.

> > As for simply using it despite the BIOS corrupting it, that might not
> > always work, the BIOS might simply over-write your state because it
> > one-sidedly declares to own the MSRs (observed behaviour).
> 
> Yes, that doesn't work. If someone else is active you have to step back.
> 
> > Its all a big clusterfuck and really the best way (IMO) is what we have
> > now to put pressure on and force the BIOS vendors to play nice.
> 
> It just results in users like Eric being screwed.  I doubt that any
> BIOS writer ever heard about it. Congratulations for a great plan.

I'd encourage you to think through this topic instead of making derisive 
comments about others ...

Thanks,

	Ingo
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