linux-kernel - Re: [PATCH v14 07/14] mm: multi-gen LRU: exploit locality in rmap

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Message-Id: <0F7CF2A7-F671-4196-B8FD-F35E9556391B@gmail.com>
Date:   Thu, 1 Sep 2022 02:18:10 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, X86 ML <x86@...nel.org>,
        page-reclaim@...gle.com, Barry Song <baohua@...nel.org>,
        Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v14 07/14] mm: multi-gen LRU: exploit locality in rmap



> On Aug 15, 2022, at 12:13 AM, Yu Zhao <yuzhao@...gle.com> wrote:
> 
> 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.

> 
> +	for (i = 0, addr = start; addr != end; i++, addr += PAGE_SIZE) {
> +		unsigned long pfn;
> +
> +		pfn = get_pte_pfn(pte[i], pvmw->vma, addr);
> +		if (pfn == -1)
> +			continue;
> +
> +		if (!pte_young(pte[i]))
> +			continue;
> +
> +		folio = 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 “if" and “continue".

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.