linux-kernel - Re: [PATCH 10/25] Calculate the alloc

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Date:	Tue, 21 Apr 2009 11:05:30 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 10/25] Calculate the alloc_flags for allocation only
	once

On Tue, Apr 21, 2009 at 06:03:25PM +0900, KOSAKI Motohiro wrote:
> > Factor out the mapping between GFP and alloc_flags only once. Once factored
> > out, it only needs to be calculated once but some care must be taken.
> > 
> > [neilb@...e.de says]
> > As the test:
> > 
> > -       if (((p->flags & PF_MEMALLOC) || unlikely(test_thread_flag(TIF_MEMDIE)))
> > -                       && !in_interrupt()) {
> > -               if (!(gfp_mask & __GFP_NOMEMALLOC)) {
> > 
> > has been replaced with a slightly weaker one:
> > 
> > +       if (alloc_flags & ALLOC_NO_WATERMARKS) {
> > 
> > we need to ensure we don't recurse when PF_MEMALLOC is set.
> 
> It seems good idea.
> 
> > +static inline int
> > +gfp_to_alloc_flags(gfp_t gfp_mask)
> > +{
> > +	struct task_struct *p = current;
> > +	int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
> > +	const gfp_t wait = gfp_mask & __GFP_WAIT;
> > +
> > +	/*
> > +	 * The caller may dip into page reserves a bit more if the caller
> > +	 * cannot run direct reclaim, or if the caller has realtime scheduling
> > +	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
> > +	 * set both ALLOC_HARDER (!wait) and ALLOC_HIGH (__GFP_HIGH).
> > +	 */
> > +	if (gfp_mask & __GFP_HIGH)
> > +		alloc_flags |= ALLOC_HIGH;
> > +
> > +	if (!wait) {
> > +		alloc_flags |= ALLOC_HARDER;
> > +		/*
> > +		 * Ignore cpuset if GFP_ATOMIC (!wait) rather than fail alloc.
> > +		 * See also cpuset_zone_allowed() comment in kernel/cpuset.c.
> > +		 */
> > +		alloc_flags &= ~ALLOC_CPUSET;
> > +	} else if (unlikely(rt_task(p)) && !in_interrupt())
> 
> wait==1 and in_interrupt==1 is never occur.
> I think in_interrupt check can be removed.
> 

Looks like it. I removed it now.

> >  	/* Atomic allocations - we can't balance anything */
> >  	if (!wait)
> >  		goto nopage;
> >  
> > +	/* Avoid recursion of direct reclaim */
> > +	if (p->flags & PF_MEMALLOC)
> > +		goto nopage;
> > +
> 
> Again. old code doesn't only check PF_MEMALLOC, but also check TIF_MEMDIE.
> 

But a direct reclaim will have PF_MEMALLOC set and doesn't care about
the value of TIF_MEMDIE with respect to recursion.

There is still a check made for TIF_MEMDIE for setting ALLOC_NO_WATERMARKS
in gfp_to_alloc_flags() so that flag is still being taken care of.

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