linux-kernel - Re: [PATCH 1.5/2] generic-smp: fix initial quiesent count.

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Date:	Sun, 15 Feb 2009 00:29:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1.5/2] generic-smp: fix initial quiesent count.


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

> On Sat, 2009-02-14 at 15:46 +0100, Peter Zijlstra wrote:
> > first compile, then send out...
> > 
> > ---
> > Subject: generic-smp: fix initial quiesent count.
> > From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date: Sat Feb 14 15:36:07 CET 2009
> > 
> > If we start with a quiesent sequence count of 0, we'll match the initial stamp
> > values of the cfd_data, and never make any progress.
> > 
> > To avoid getting stuck in this situation, start out with an increased quiesent
> > sequence count.
> 
> OK, so this is not going to fix the problem in generic. Its still
> possible to end up with the original issue.
> 
> We'd need a callback list to free used entries, much like regular RCU,
> this quiesent sequence count's wrapping just seems too brittle.

Ok, given that your series was hanging on a number of testboxes here,
lets go with my original simplification?

The broadcasting-to-a-lot-of-CPUs case is going to suck on any seriously
large system no matter what. It's single-IPI and few-IPI performance that
matters mostly.

	Ingo
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