linux-kernel - Re: [PATCH 0/3] TLB flush multiple pages per IPI v5

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Date:	Tue, 9 Jun 2015 23:35:09 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	H Peter Anvin <hpa@...or.com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] TLB flush multiple pages per IPI v5

On Tue, Jun 09, 2015 at 11:32:03PM +0100, Mel Gorman wrote:
> On Tue, Jun 09, 2015 at 02:54:01PM -0700, Linus Torvalds wrote:
> > On Tue, Jun 9, 2015 at 2:14 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> > >
> > > The 0 cycle TLB miss was also interesting.  It goes back up to something
> > > reasonable if I put the mb()/mfence's back.
> > 
> > So I've said it before, and I'll say it again: Intel does really well
> > on TLB fills.
> > 
> > The reason is partly historical, with Win95 doing a ton of TLB
> > invalidation (I think every single GDI call ended up invalidating the
> > TLB, so under some important Windows benchmarks of the time, you
> > literally had a TLB flush every 10k instructions!).
> > 
> > But partly it is because people are wrong in thinking that TLB fills
> > have to be slow. There's a lot of complete garbage RISC machines where
> > the TLB fill took forever, because in the name of simplicity it would
> > stop the pipeline and often be done in SW.
> > 
> > The zero-cycle TLB fill is obviously a bit optimistic, but at the same
> > time it's not completely insane. TLB fills can be prefetched, and the
> > table walker can hit the cache, if you do them right. And Intel mostly
> > does.
> > 
> > So the normal full (non-global) TLB fill really is fairly cheap. It's
> > been optimized for, and the TLB gets re-filled fairly efficiently. I
> > suspect that it's really the case that doing more than just a couple
> > of single-tlb flushes is a complete waste of time: the flushing takes
> > longer than re-filling the TLB well.
> > 
> 
> I expect I'll do another revision of the series after 4.2-rc1 as it's way
> too close to 4.1's release. When that happens, I'll drop patch 4 and leave
> just the full non-global flush patch. In the event there is an architecture
> that really cares about the refill cost or we find that there is a corner
> case where the TLB refill hurts then patch 4 will be in the mail archives
> to consider for rebase and testing.
> 

To be clear I meant patch 4 from v6 of the series will be dropped. In that
series, patch 2 does a full non-global flush and patch 4 does the per-pfn
tracking with multiple single page frame flushes.

-- 
Mel Gorman
SUSE Labs
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