linux-kernel - Re: [PATCH] v4 Teach RCU that idle task is not quiscent state at boot

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Message-ID: <20090225173649.GF6797@linux.vnet.ibm.com>
Date:	Wed, 25 Feb 2009 09:36:49 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	vegard.nossum@...il.com, stable@...nel.org,
	akpm@...ux-foundation.org, penberg@...helsinki.fi
Subject: Re: [PATCH] v4 Teach RCU that idle task is not quiscent state at
	boot

On Wed, Feb 25, 2009 at 06:18:42PM +0100, Nick Piggin wrote:
> On Wed, Feb 25, 2009 at 09:12:49AM -0800, Paul E. McKenney wrote:
> > On Wed, Feb 25, 2009 at 05:26:45PM +0100, Nick Piggin wrote:
> > > I wonder if you couldn't leave cpu_rq(cpu)->idle as NULL until right
> > > up to the point where it actually becomes the "idle" thread? This
> > > would make idle_cpu() more truthful for all other callers in early
> > > boot code too. And rcupdate shouldn't need any changes (except the
> > > num_online_cpus() == 1 shortcut probably remains as a nice opt).
> > 
> > It looked to me that the idle task initialization was moved early
> > to allow interrupt handlers to see a more normal environment, but I
> > could easily be confused here.
> 
> I think we might be OK. I think interrupt handlers shouldn't care
> so much about whether the cpu is idle or not (and seeing !idle
> here would be more accurate anyway).

My impression is that they mostly care about the CPU appearing to be
running in the context of a task.  I suppose one could create an extra
task for this, or leave rq->idle NULL...  But I don't have a full set
of architectures, let alone devices, with which to test such a change.

							Thanx, Paul
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