linux-kernel - Re: [RFC PATCH 1/2] zap_pte_range: update addr when forcing flush after TLB batching faiure

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Date:	Wed, 29 Oct 2014 08:16:34 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Subject: Re: [RFC PATCH 1/2] zap_pte_range: update addr when forcing flush
 after TLB batching faiure

On Tue, 2014-10-28 at 09:25 -0700, Linus Torvalds wrote:

> > Since we have hardware broadcasting of TLB invalidations on ARM, it is
> > in our interest to keep the number of outstanding operations as small as
> > possible, particularly on large systems where we don't get the targetted
> > shootdown with a single message that you can perform using IPIs (i.e.
> > you can only broadcast to all or no CPUs, and that happens for each pte).
> 
> Do you seriously *have* to broadcast for each pte?

We do too, in current CPUs at least, it's sad ...

> Because that is quite frankly moronic.  We batch things up in software
> for a real good reason: doing things one entry at a time just cannot
> ever scale. At some point (and that point is usually not even very far
> away), it's much better to do a single invalidate over a range. The
> cost of having to refill the TLB's is *much* smaller than the cost of
> doing tons of cross-CPU invalidates.
> 
> That's true even for the cases where we track the CPU's involved in
> that mapping, and only invalidate a small subset. With a "all CPU's
> broadcast", the cross-over point must be even smaller. Doing thousands
> of CPU broadcasts is just crazy, even if they are hw-accelerated.
> 
> Can't you just do a full invalidate and a SW IPI for larger ranges?

For us, this would be great except ... we can potentially have other
agents with an MMU that only support snooping of the broadcasts...

> And as mentioned, true sparse mappings are actually fairly rare, so
> making extra effort (and data structures) to have individual ranges
> sounds crazy.
> 
> Is this some hw-enforced thing? You really can't turn off the
> cross-cpu-for-each-pte braindamage?

Cheers,
Ben.

>                          Linus
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