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

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Date:   Wed, 2 Aug 2017 00:59:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
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 Tue, Aug 01, 2017 at 06:48:20PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 01, 2017 at 05:44:14PM +0100, Will Deacon wrote:
> > On Tue, Aug 01, 2017 at 06:39:03PM +0200, Peter Zijlstra wrote:
> > > Still this is all rather unsatisfactory. Either we should define
> > > flush_tlb*() to imply a barrier when its not a no-op (sparc64/ppc-hash)
> > > or simply make clear_tlb_flush_pending() an smp_store_release().
> > > 
> > > I prefer the latter option.
> > > 
> > > Opinions?
> > 
> > I prefer the latter option too, since I'd like to relax the arm64 TLB
> > flushing to have weaker barriers for the local case. Granted, that doesn't
> > break the NUMA migration code, but it would make the barrier semantics of
> > the TLB invalidation routines even more subtle if we were to define them
> > generally.
> 
> Another 'fun' question, is smp_mb() strong enough to order against the
> TLB invalidate? Because we really want to clear this flag _after_.
> 
> PowerPC for example uses PTESYNC before the TBLIE, so does a SYNC after
> work? Ben?

>From what I gather it is not. You have TLBSYNC for it. So the good news
is that PPC-radix does all that and is fully serialized on the tlb
flush. Not sure for the PPC-hash case.

At the same time, smp_mb() is not sufficient on ARM either, they need a
DSB barrier on both ends.

So are we going to mandate tlb flush implementations are completely
ordered ?