linux-kernel - Re: post-3.18 performance regression in TLB flushing code

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Date:	Wed, 17 Dec 2014 16:53:10 +0000
From:	Will Deacon <will.deacon@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Hansen <dave@...1.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Michal Simek <monstr@...str.eu>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: post-3.18 performance regression in TLB flushing code

On Wed, Dec 17, 2014 at 04:28:23PM +0000, Linus Torvalds wrote:
> On Wed, Dec 17, 2014 at 2:08 AM, Will Deacon <will.deacon@....com> 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.

I guess I was being overly cautious in case tlb_flush_mmu_tlbonly grows
additional users, but you're right.

> 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")?

Looks fine to me... Dave?

Will

> 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);
>  }

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