linux-kernel - Re: [this_cpu_xx V4 13/20] this_cpu

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Message-ID: <20091006170333.GE18185@csn.ul.ie>
Date:	Tue, 6 Oct 2009 18:03:33 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, mingo@...e.hu,
	rusty@...tcorp.com.au, Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [this_cpu_xx V4 13/20] this_cpu_ops: page allocator conversion

On Tue, Oct 06, 2009 at 12:34:56PM -0400, Christoph Lameter wrote:
> On Tue, 6 Oct 2009, Mel Gorman wrote:
> 
> > > -	local_irq_save(flags);
> > > -	pcp = &this_cpu_ptr(zone->pageset)->pcp;
> > >  	migratetype = get_pageblock_migratetype(page);
> > >  	set_page_private(page, migratetype);
> > >  	if (unlikely(wasMlocked))
> >
> > Why did you move local_irq_save() ? It should have stayed where it was
> > because VM counters are updated under the lock. Only the this_cpu_ptr
> > should be moving.
> 
> The __count_vm_event()?

and the __dec_zone_page_state within free_page_mlock(). However, it's already
atomic so it shouldn't be a problem.

> VM counters may be incremented in a racy way if
> convenient. x86 usually produces non racy code (and with this patchset
> will always produce non racy code) but f.e. IA64 has always had racy
> updates. I'd rather shorted the irq off section.
> 

The count_vm_event is now racier than it was and no longer symmetric with
the PGALLOC counting which still happens with IRQs disabled. The assymetry
could look very strange if there are a lot more frees than allocs for example
because the raciness between the counters is difference.

While I have no problem as such with the local_irq_save() moving (although
I would like PGFREE and PGALLOC to be accounted both with or without IRQs
enabled), I think it deserves to be in a patch all to itself and not hidden
in an apparently unrelated change.

> See the comment in vmstat.h.
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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