linux-kernel - Re: [PATCH, RFC] hacks to allow -rt to run kernbench on POWER

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Date:	Mon, 29 Oct 2007 13:26:17 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-kernel@...r.kernel.org, tony@...eyournoodle.com,
	paulus@...ba.org, dino@...ibm.com, tytso@...ibm.com,
	dvhltc@...ibm.com, antonb@...ibm.com, rostedt@...dmis.org,
	niv@...ibm.com
Subject: Re: [PATCH, RFC] hacks to allow -rt to run kernbench on POWER

On Tue, Oct 30, 2007 at 07:07:48AM +1100, Benjamin Herrenschmidt wrote:
> 
> On Mon, 2007-10-29 at 11:50 -0700, Paul E. McKenney wrote:
> > Hello!
> > 
> > A few random patches that permit POWER to pass kernbench on -rt.
> > Many of these have more focus on expediency than care for correctness,
> > so might best be thought of as workarounds than as complete solutions.
> > There are still issues not addressed by this patch, including:
> > 
> > o	kmem_cache_alloc() from non-preemptible context during
> > 	bootup (xics_startup() building the irq_radix_revmap()).
> > 
> > o	unmap_vmas() freeing pages with preemption disabled.
> > 	Might be able to address this by linking the pages together,
> > 	then freeing them en masse after preemption has been re-enabled,
> > 	but there is likely a better approach.
> > 
> > Thoughts?
> 
> I see a lot of case where you add preempt_disable/enable around areas
> that have the PTE lock held...
> 
> So in -rt, spin_lock doesn't disable preempt ? I'm a bit worried...
> there are some strong requirements that anything within that lock is not
> preempted, so zap_pte_ranges() is the obvious ones but all of them would
> need to be addressed.

Right in one!  One of the big changes in -rt is that spinlock critical
sections (and RCU read-side critical sections, for that matter) are
preemptible under CONFIG_PREEMPT_RT.

And I agree that this patchset will have missed quite a few places where
additional changes are required.  Hence the word "including" above, rather
than something like "specifically".  ;-)

							Thanx, Paul
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