linux-kernel - Re: [RFC PATCH 2/4] mm: kpromoted: Hot page info collection and promotion daemon

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Message-ID: <Z9ho77H94jsLHDNu@gourry-fedora-PF4VCD3F>
Date: Mon, 17 Mar 2025 14:24:47 -0400
From: Gregory Price <gourry@...rry.net>
To: Bharata B Rao <bharata@....com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [RFC PATCH 2/4] mm: kpromoted: Hot page info collection and
 promotion daemon

On Mon, Mar 17, 2025 at 09:52:29PM +0530, Bharata B Rao wrote:
> > 
> > > kpromoted_recorded_accesses 960620 /* Number of recorded accesses */
> > > kpromoted_recorded_hwhints 960620  /* Nr accesses via HW hints, IBS in this
> > > case */
> > > kpromoted_recorded_pgtscans 0
> > > kpromoted_record_toptier 638006 /* Nr toptier accesses */
> > > kpromoted_record_added 321234 /* Nr (CXL) accesses that are tracked */
> > > kpromoted_record_exists 1380
> > > kpromoted_mig_right_node 0
> > > kpromoted_mig_non_lru 226
> > > kpromoted_mig_lru_active 47 /* Number of accesses considered for promotion
> > > as determined by folio_test_active() check */
> 
> However disabling demotion has no impact on this number (and hence the
> folio_test_active() check)
>

I've been mulling over what's likely to occur when the Low but not Min
watermark is hit and reclaim is invoked but without demotion enabled.

I'm wonder if kswapd pushes things like r/o pagecache out, only to have
them faulted back into CXL later, while new allocations stick on the
main memory.

You might try MPOL_PREFERRED with CXL node as the target instead of bind
w/ the local node to at least make sure the system is actually
identifying hotness correctly.

~Gregory