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Message-ID: <20230817081631.GA48308@system.software.com>
Date: Thu, 17 Aug 2023 17:16:31 +0900
From: Byungchul Park <byungchul@...com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
kernel_team@...ynix.com, akpm@...ux-foundation.org,
namit@...are.com, xhao@...ux.alibaba.com,
mgorman@...hsingularity.net, hughd@...gle.com, willy@...radead.org,
david@...hat.com
Subject: Re: [RFC 2/2] mm: Defer TLB flush by keeping both src and dst folios
at migration
On Tue, Aug 15, 2023 at 09:27:26AM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@...com> writes:
>
> > Implementation of CONFIG_MIGRC that stands for 'Migration Read Copy'.
> >
> > We always face the migration overhead at either promotion or demotion,
> > while working with tiered memory e.g. CXL memory and found out TLB
> > shootdown is a quite big one that is needed to get rid of if possible.
> >
> > Fortunately, TLB flush can be defered or even skipped if both source and
> > destination of folios during migration are kept until all TLB flushes
> > required will have been done, of course, only if the target PTE entries
> > have read only permission, more precisely speaking, don't have write
> > permission. Otherwise, no doubt the folio might get messed up.
> >
> > To achieve that:
> >
> > 1. For the folios that have only non-writable TLB entries, prevent
> > TLB flush by keeping both source and destination of folios during
> > migration, which will be handled later at a better time.
> >
> > 2. When any non-writable TLB entry changes to writable e.g. through
> > fault handler, give up CONFIG_MIGRC mechanism so as to perform
> > TLB flush required right away.
> >
> > 3. TLB flushes can be skipped if all TLB flushes required to free the
> > duplicated folios have been done by any reason, which doesn't have
> > to be done from migrations.
> >
> > 4. Adjust watermark check routine, __zone_watermark_ok(), with the
> > number of duplicated folios because those folios can be freed
> > and obtained right away through appropreate TLB flushes.
> >
> > 5. Perform TLB flushes and free the duplicated folios pending the
> > flushes if page allocation routine is in trouble due to memory
> > pressure, even more aggresively for high order allocation.
>
> Is the optimization restricted for page migration only? Can it be used
> for other places? Like page reclaiming?
I don't get you but if I'm missing something here and your idea is
something good, then it can be a future work.
Byungchul
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