Re: [PATCH 09/22] Calculate the alloc_flags for allocation only once

Mel Gorman
2009-04-24T10:47:30Z

On Thu, Apr 23, 2009 at 03:52:16PM -0700, Andrew Morton wrote:
> On Wed, 22 Apr 2009 14:53:14 +0100
> Mel Gorman <mel@csn.ul.ie> 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) {
> >
> > Without care, this would allow recursion into the allocator via direct
> > reclaim. This patch ensures we do not recurse when PF_MEMALLOC is set
> > but TF_MEMDIE callers are now allowed to directly reclaim where they
> > would have been prevented in the past.
> >
> > ...
> >
> > +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)))
> > + alloc_flags |= ALLOC_HARDER;
> > +
> > + if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> > + if (!in_interrupt() &&
> > + ((p->flags & PF_MEMALLOC) ||
> > + unlikely(test_thread_flag(TIF_MEMDIE))))
> > + alloc_flags |= ALLOC_NO_WATERMARKS;
> > + }
> > +
> > + return alloc_flags;
> > +}
>
> hm. Was there a particular reason for the explicit inline?
>

Only because it was known there was only one caller.

> It's OK as it stands, but might become suboptimal if we later add a
> second caller?
>

This is true. As it is also in the slowpath and only called once, the
following patch should make no difference to performance but potentially
avoid a mistake later.

=======
Uninline gfp_to_alloc_flags() in the page allocator slow path

gfp_to_alloc_flags() is in the slowpath but inlined. While there is only one
caller now, a future second call would add suprising text-bloat. Uninline
it now to avoid surprises later as it should have no performance impact.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
mm/page_alloc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1c60141..f08b4cb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1639,7 +1639,7 @@ void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
wakeup_kswapd(zone, order);
}

-static inline int
+static int
gfp_to_alloc_flags(gfp_t gfp_mask)
{
struct task_struct *p = current;