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

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Message-ID: <CA+55aFyVxOw0upa=At6MmiNYEHzfPz4rE5bZUBCs9h4vKGh1iA@mail.gmail.com>
Date:	Wed, 17 Dec 2014 08:28:23 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Will Deacon <will.deacon@....com>
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 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.

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

View attachment "patch.diff" of type "text/plain" (714 bytes)