Commit-ID: b13b1d2d8692b437203de7a404c6b809d2cc4d99 Gitweb: http://git.kernel.org/tip/b13b1d2d8692b437203de7a404c6b809d2cc4d99 Author: Shaohua Li <[email protected]> AuthorDate: Tue, 8 Apr 2014 15:58:09 +0800 Committer: Ingo Molnar <[email protected]> CommitDate: Wed, 16 Apr 2014 08:57:08 +0200
x86/mm: In the PTE swapout page reclaim case clear the accessed bit instead of flushing the TLB We use the accessed bit to age a page at page reclaim time, and currently we also flush the TLB when doing so. But in some workloads TLB flush overhead is very heavy. In my simple multithreaded app with a lot of swap to several pcie SSDs, removing the tlb flush gives about 20% ~ 30% swapout speedup. Fortunately just removing the TLB flush is a valid optimization: on x86 CPUs, clearing the accessed bit without a TLB flush doesn't cause data corruption. It could cause incorrect page aging and the (mistaken) reclaim of hot pages, but the chance of that should be relatively low. So as a performance optimization don't flush the TLB when clearing the accessed bit, it will eventually be flushed by a context switch or a VM operation anyway. [ In the rare event of it not getting flushed for a long time the delay shouldn't really matter because there's no real memory pressure for swapout to react to. ] Suggested-by: Linus Torvalds <[email protected]> Signed-off-by: Shaohua Li <[email protected]> Acked-by: Rik van Riel <[email protected]> Acked-by: Mel Gorman <[email protected]> Acked-by: Hugh Dickins <[email protected]> Acked-by: Johannes Weiner <[email protected]> Cc: [email protected] Cc: Peter Zijlstra <[email protected]> Link: http://lkml.kernel.org/r/[email protected] [ Rewrote the changelog and the code comments. ] Signed-off-by: Ingo Molnar <[email protected]> --- arch/x86/mm/pgtable.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-) diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c index c96314a..0004ac7 100644 --- a/arch/x86/mm/pgtable.c +++ b/arch/x86/mm/pgtable.c @@ -399,13 +399,20 @@ int pmdp_test_and_clear_young(struct vm_area_struct *vma, int ptep_clear_flush_young(struct vm_area_struct *vma, unsigned long address, pte_t *ptep) { - int young; - - young = ptep_test_and_clear_young(vma, address, ptep); - if (young) - flush_tlb_page(vma, address); - - return young; + /* + * On x86 CPUs, clearing the accessed bit without a TLB flush + * doesn't cause data corruption. [ It could cause incorrect + * page aging and the (mistaken) reclaim of hot pages, but the + * chance of that should be relatively low. ] + * + * So as a performance optimization don't flush the TLB when + * clearing the accessed bit, it will eventually be flushed by + * a context switch or a VM operation anyway. [ In the rare + * event of it not getting flushed for a long time the delay + * shouldn't really matter because there's no real memory + * pressure for swapout to react to. ] + */ + return ptep_test_and_clear_young(vma, address, ptep); } #ifdef CONFIG_TRANSPARENT_HUGEPAGE -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
