[PATCH] fix free swap cache latency

Lee Revell reported 28ms latency when process with lots of swapped memory
exits.

2.6.15 introduced a latency regression when unmapping: in accounting the
zap_work latency breaker, pte_none counted 1, pte_present PAGE_SIZE, but a
swap entry counted nothing at all.  We think of pages present as the slow
case, but Lee's trace shows that free_swap_and_cache's radix tree lookup
can make a lot of work - and we could have been doing it many thousands of
times without a latency break.

Move the zap_work update up to account swap entries like pages present.
This does account non-linear pte_file entries, and unmap_mapping_range
skipping over swap entries, by the same amount even though they're quick:
but neither of those cases deserves complicating the code (and they're
treated no worse than they were in 2.6.14).

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/mm/memory.c b/mm/memory.c
index 9abc6008544baae37d9d88a4416a4ec81e966132..85e80a57db29e2412ef766268996ec005c3e16c8 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -623,11 +623,12 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
                        (*zap_work)--;
                        continue;
                }
+
+               (*zap_work) -= PAGE_SIZE;
+
                if (pte_present(ptent)) {
                        struct page *page;
 
-                       (*zap_work) -= PAGE_SIZE;
-
                        page = vm_normal_page(vma, addr, ptent);
                        if (unlikely(details) && page) {
                                /*