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

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Date:   Thu, 03 Aug 2017 08:54:40 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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, 2017-08-02 at 10:02 +0100, Will Deacon wrote:
> > > 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]

We do that on powerpc too, though there are ongoing questions a to
whether an smp_mb() after setting the mask bit in switch_mm is
sufficient vs. prefetch brining entries in the TLB after the context is
switched. But that's a powerpc specific issue. Nick Piggin is working
on sorting that out.

Cheers,
Ben.