linux-kernel - Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush

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Date:   Wed, 2 Aug 2017 10:02:21 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        torvalds@...ux-foundation.org, oleg@...hat.com,
        paulmck@...ux.vnet.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu, Mel Gorman <mgorman@...e.de>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush_pending()

On Wed, Aug 02, 2017 at 10:51:11AM +0200, Peter Zijlstra wrote:
> On Wed, Aug 02, 2017 at 09:43:50AM +0100, Will Deacon wrote:
> > On Wed, Aug 02, 2017 at 09:15:23AM +0100, Will Deacon wrote:
> 
> > > I really think we should avoid defining TLB invalidation in terms of
> > > smp_mb() because it's a lot more subtle than that.
> > 
> > Another worry I have here is with architectures that can optimise the
> > "only need to flush the local TLB" case. For example, this version of 'R':
> > 
> > 
> > P0:
> > WRITE_ONCE(x, 1);
> > smp_mb();
> > WRITE_ONCE(y, 1);
> > 
> > P1:
> > WRITE_ONCE(y, 2);
> > flush_tlb_range(...);  // Only needs to flush the local TLB
> > r0 = READ_ONCE(x);
> > 
> > 
> > It doesn't seem unreasonable to me for y==2 && r0==0 if the
> > flush_tlb_range(...) ends up only doing local invalidation. As a concrete
> > example, imagine a CPU with a page table walker that can snoop the local
> > store-buffer. Then, the local flush_tlb_range in P1 only needs to progress
> > the write to y as far as the store-buffer before it can invalidate the local
> > TLB. Once the TLB is invalidated, it can read x knowing that the translation
> > is up-to-date wrt the page table, but that read doesn't need to wait for
> > write to y to become visible to other CPUs.
> > 
> > So flush_tlb_range is actually weaker than smp_mb in some respects, yet the
> > flush_tlb_pending stuff will still work correctly.
> 
> So while I think you're right, and we could live with this, after all,
> if we know the mm is CPU local, there shouldn't be any SMP concerns wrt
> its page tables. Do you really want to make this more complicated?

It gives us a nice performance lift on arm64 and I have a patch...[1]

Will

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/commit/?h=aarch64/devel&id=1c7cf53658f0fa16338d1f8406285ae28fd5f616