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

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Message-Id: <20240522155641.a726c5cd3b25aa23e861045d@linux-foundation.org>
Date: Wed, 22 May 2024 15:56:41 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Byungchul Park <byungchul@...com>
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 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
happens ;)