linux-kernel - Re: [RESEND PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

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Date: Thu, 23 May 2024 11:05:39 +0900
From: Byungchul Park <byungchul@...com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, ying.huang@...el.com, vernhao@...cent.com,
	mgorman@...hsingularity.net, hughd@...gle.com, willy@...radead.org,
	david@...hat.com, peterz@...radead.org, luto@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, rjgolo@...il.com
Subject: Re: [RESEND PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb
 numbers over 90%

On Wed, May 22, 2024 at 03:56:41PM -0700, Andrew Morton wrote:
> On Mon, 20 May 2024 11:17:22 +0900 Byungchul Park <byungchul@...com> wrote:
> 
> > While I'm working with a tiered memory system e.g. CXL memory, I have
> > been facing migration overhead esp. tlb shootdown on promotion or
> > demotion between different tiers.  Yeah..  most tlb shootdowns on
> > migration through hinting fault can be avoided thanks to Huang Ying's
> > work, commit 4d4b6d66db ("mm,unmap: avoid flushing tlb in batch if PTE
> > is inaccessible").  See the following link for more information:
> > 
> > https://lore.kernel.org/lkml/20231115025755.GA29979@system.software.com/
> > 
> > However, it's only for migration through hinting fault.  I thought it'd
> > be much better if we have a general mechanism to reduce all the tlb
> > numbers that we can apply to any unmap code, that we normally believe
> > tlb flush should be followed.
> > 
> > I'm suggesting a new mechanism, LUF(Lazy Unmap Flush), defers tlb flush
> > until folios that have been unmapped and freed, eventually get allocated
> > again.  It's safe for folios that had been mapped read-only and were
> > unmapped, since the contents of the folios don't change while staying in
> > pcp or buddy so we can still read the data through the stale tlb entries.
> 
> Version 10 and no reviewed-by's or acked-by's.  Reviewing the review
> history isn't helped by the change in the naming of the patch series.
> 
> Seems that you're measuring a ~5% overall speedup in a realistic
> workload?  That's nice.
> 
> I'll defer this for a week or so to see what reviewers have to say.  If
> "nothing", please poke me and I guess I'll merge it up to see what

I will poke you and will be ready for that ;)

	Byungchul

> happens ;)