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Subject: Re: [PATCH v14 07/14] mm: multi-gen LRU: exploit locality in rmap
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20220815071332.627393-8-yuzhao@google.com>
Date:   Thu, 1 Sep 2022 02:18:10 -0700
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Aneesh Kumar <aneesh.kumar@linux.ibm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
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References: <20220815071332.627393-1-yuzhao@google.com>
 <20220815071332.627393-8-yuzhao@google.com>
To:     Yu Zhao <yuzhao@google.com>
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> On Aug 15, 2022, at 12:13 AM, Yu Zhao <yuzhao@google.com> wrote:
>=20
> Searching the rmap for PTEs mapping each page on an LRU list (to test
> and clear the accessed bit) can be expensive because pages from
> different VMAs (PA space) are not cache friendly to the rmap (VA
> space). For workloads mostly using mapped pages, searching the rmap
> can incur the highest CPU cost in the reclaim path.

Impressive work. Sorry if my feedback is not timely.

Just one minor point for thought, that can be left for a later cleanup.

>=20
> +	for (i =3D 0, addr =3D start; addr !=3D end; i++, addr +=3D =
PAGE_SIZE) {
> +		unsigned long pfn;
> +
> +		pfn =3D get_pte_pfn(pte[i], pvmw->vma, addr);
> +		if (pfn =3D=3D -1)
> +			continue;
> +
> +		if (!pte_young(pte[i]))
> +			continue;
> +
> +		folio =3D get_pfn_folio(pfn, memcg, pgdat);
> +		if (!folio)
> +			continue;
> +
> +		if (!ptep_test_and_clear_young(pvmw->vma, addr, pte + =
i))
> +			continue;
> +

You have already checked that the PTE is old (not young), so this check
seems redundant. I do not see a way in which the access-bit can be =
cleared
since you hold the ptl. IOW, there is no need for the =E2=80=9Cif" and =
=E2=80=9Ccontinue".

Makes me also wonder whether having a separate ptep_clear_young() can
slightly help, since anyhow the access-bit is more of an estimation,
and having a separate ptep_clear_young() can enable optimizations.

On x86, for instance, if the PTE is dirty, we may be able to clear the
access-bit without an atomic operation, which should be faster.