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

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Message-ID: <20110325095931.GA31903@elte.hu>
Date:	Fri, 25 Mar 2011 10:59:31 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jack Steiner <steiner@....com>,
	Jan Beulich <JBeulich@...ell.com>,
	Borislav Petkov <bp@...64.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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


* Eric Dumazet <eric.dumazet@...il.com> wrote:

> Le vendredi 25 mars 2011 à 10:32 +0100, Ingo Molnar a écrit :
> > * Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> > > Le vendredi 25 mars 2011 à 00:56 +0100, Andi Kleen a écrit :
> > > > > never EVER seen any good explanation of why that particular sh*t
> > > > > argument would b true. It seems to be purely about politics, where
> > > > > some idiotic vendor (namely HP) has convinced Intel that they really
> > > > > need it. To the point where some engineers seem to have bought into
> > > > > the whole thing and actually believe that fairy tale ("firmware can do
> > > > > better" - hah! They must be feeding people some bad drugs at the
> > > > > cafeteria)
> > > > 
> > > > For the record I don't think it's a good idea for the BIOS to do 
> > > > this (and I'm not aware of any engineer who does),  
> > > > but I think Linux should do better than just disabling PMU use when 
> > > > this happens.
> > > > 
> > > > However I suspect taking over SCI would cause endless problems
> > > > and is very likely not a good idea.
> > > 
> > > I tried many different changes in BIOS and all failed (the machine is
> > > damn slow at boot, this takes age).
> > > 
> > > I am stuck :(
> > 
> > Could you please try the patch below?
> 
> This obviously works, [...]

Congrats to your now much-improved perf experience! :-)

> [...] but you probably need to make a full pass to make sure we dont have a 
> MSR failure -this should return false in this case.

Wanted to keep this patch simple - we are not really hitting MSR failure cases 
in practice, and by getting the message the user is at least warned that 
*something* is amiss.

Thanks,

	Ingo
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