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[24.6.216.183]) by smtp.gmail.com with ESMTPSA id bg8-20020a056a02010800b0042a5e8486a1sm4782830pgb.42.2022.09.01.02.18.12 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Sep 2022 02:18:15 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.120.41.1.1\)) Subject: Re: [PATCH v14 07/14] mm: multi-gen LRU: exploit locality in rmap From: Nadav Amit In-Reply-To: <20220815071332.627393-8-yuzhao@google.com> Date: Thu, 1 Sep 2022 02:18:10 -0700 Cc: Andrew Morton , Andi Kleen , Aneesh Kumar , Catalin Marinas , Dave Hansen , Hillf Danton , Jens Axboe , Johannes Weiner , Jonathan Corbet , Linus Torvalds , Matthew Wilcox , Mel Gorman , Michael Larabel , Michal Hocko , Mike Rapoport , Peter Zijlstra , Tejun Heo , Vlastimil Babka , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, LKML , Linux MM , X86 ML , page-reclaim@google.com, Barry Song , Brian Geffon , Jan Alexander Steffens , Oleksandr Natalenko , Steven Barrett , Suleiman Souhlal , Daniel Byrne , Donald Carr , =?utf-8?Q?Holger_Hoffst=C3=A4tte?= , Konstantin Kharlamov , Shuang Zhai , Sofia Trinh , Vaibhav Jain Content-Transfer-Encoding: quoted-printable Message-Id: <0F7CF2A7-F671-4196-B8FD-F35E9556391B@gmail.com> References: <20220815071332.627393-1-yuzhao@google.com> <20220815071332.627393-8-yuzhao@google.com> To: Yu Zhao X-Mailer: Apple Mail (2.3696.120.41.1.1) ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1662023897; a=rsa-sha256; cv=none; b=Rm6ROBs132YviDM4fKpCwoUCXRP5xeySwQwe5s6YCuUAFit/GskToxOkQKbW5VN2ENWq98 J4BJADiBrwlVkubNNm1R/FGOnQTuSz1y4884G3Bdb5weX/I3LqSaq1UGZm05VLyKJYEfm0 EWQpP7gBAL5tVF1xIrHJNtwDLxv3Kjs= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=aMwYpy2K; spf=pass (imf26.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.210.177 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1662023897; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=EHj+UQ7Tu6usfmBtBbDH78MoNwVO7qgjOcn++ecvlvY=; b=PoJQKlMmmCUi4i2k6jod/jua12xiJgoY2JosxPu1lIDLLimRCqFdo1ZAmpNbF4ng4b0/Co 5uv/4YsevXc+srv4YGr2NyCi3iAuf4zovubtMvsIk5EJPb4iM1G54PkM9nz4FuWGxRjxC0 XtII2kQHwxW7esUlcQL1azMn42+Jby4= X-Rspamd-Queue-Id: 7BACA14006E Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=aMwYpy2K; spf=pass (imf26.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.210.177 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspam-User: X-Rspamd-Server: rspam01 X-Stat-Signature: gdebxextn7up1egpdqn8z7qreezfibsm X-HE-Tag: 1662023897-586588 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Aug 15, 2022, at 12:13 AM, Yu Zhao 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.