Re: post-3.18 performance regression in TLB flushing code

[Linus Torvalds]

On Wed, Dec 17, 2014 at 2:08 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
>
> I think there are a couple of things you could try to see if that 2% comes
> back:
>
>   * Revert the patch and try the one here [1] instead (which only does part
>     (1) of the above).
>
> -- or --
>
>   * Instead of adding the tlb->end check to tlb_flush_mmu, add it to
>     tlb_flush_mmu_free

or just move the check back to tlb_flush_mmu() where it belongs.

I don't see why you moved it to "tlb_flush_mmu_tlbonly()" in the first
place, or why you'd now want to add it to tlb_flush_mmu_free().

Both of those helper functions have two callers:

 - tlb_flush_mmu(). Doing it here (instead of in the helper functions)
is the right thing to do

 - the "force_flush" case: we know we have added at least one page to
the TLB state so checking for it is pointless.

So I'm not seeing why you wanted to do it in tlb_flush_mmu_tlbonly(),
and now add it to tlb_flush_mmu_free(). That seems bogus.

So why not just this trivial patch, to make the logic be the same it
used to be (just using "end > 0" instead of the old "need_flush")?

                           Linus
 mm/memory.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index c3b9097251c5..6efe36a998ba 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -235,9 +235,6 @@ void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm, unsigned long
 
 static void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
 {
-	if (!tlb->end)
-		return;
-
 	tlb_flush(tlb);
 	mmu_notifier_invalidate_range(tlb->mm, tlb->start, tlb->end);
 #ifdef CONFIG_HAVE_RCU_TABLE_FREE
@@ -259,6 +256,9 @@ static void tlb_flush_mmu_free(struct mmu_gather *tlb)
 
 void tlb_flush_mmu(struct mmu_gather *tlb)
 {
+	if (!tlb->end)
+		return;
+
 	tlb_flush_mmu_tlbonly(tlb);
 	tlb_flush_mmu_free(tlb);
 }