linux-kernel - Re: per-cpu operation madness vs validation

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Date:	Wed, 27 Jul 2011 19:33:14 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Christoph Lameter <cl@...ux.com>
cc:	Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, eric.dumazet@...il.com
Subject: Re: per-cpu operation madness vs validation

On Wed, 27 Jul 2011, Christoph Lameter wrote:
> On Wed, 27 Jul 2011, Peter Zijlstra wrote:
> > All this is simply about de-obfuscating all per-cpu assumptions. This is
> > about verification and traceability/debuggability.
> 
> Ok verification  and traceability are good but they should not
> be in the way of making core functionality high performance and low
> latency.
> 
> The key issue is that the -rt kernel has always had grave issues with
> performance when it comes to per cpu data access. Solving that by forcing
> the kernel to go slow it not the right approach.

Nobody want's the kernel to go slow. All we want and we consider that
also a benefit for mainline is: proper annotation of the per cpu data
access, like we have for RCU and for locking.

Right now everything else than the "atomic" this_cpu stuff needs
protection of some sort, but it's nowhere documented and we have no
way to prove the correctness.

That has absolutely nothing to do with -rt. We already had cases in
mainline where per cpu data structures were accessed without or with
wrong protections because the logic changed over time or people made
the wrong assumptions.

For -rt this lack of documentation and the lack of verification,
debugability and traceability is a major PITA, but that's true for
non-rt as well, just the PITA is gradually smaller and the bugs which
are there today are just extremly hard to trigger.

And Peters idea of per_cpu_lock*() annotations will boil down to the
exact same thing which is there today when you compile the kernel w/o
lockdep enabled for per_cpu data correctness. We don't want to change
anything or impose any slowness, we just want a proper way to document
and verify that maze. That's really not too much of a request.

Thanks,

	tglx

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