Re: [PATCH 10/25] Calculate the alloc_flags for allocation only once

Mel Gorman
2009-04-21T10:05:29Z

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@suse.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