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Fri, 28 Aug 2026 07:43:00 +0000 X-Mizu-Trace-ID: 69c5c08b3e19d467 X-Migadu-Flow: FLOW_OUT Date: Fri, 28 Aug 2026 15:42:56 +0800 From: Baoquan He To: Barry Song Cc: Baoquan He , linux-mm@kvack.org, akpm@linux-foundation.org, david@kernel.org, rostedt@goodmis.org, mhiramat@kernel.org, kasong@tencent.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, baolin.wang@linux.alibaba.com, hannes@cmpxchg.org Subject: Re: [PATCH 0/9] mm/mglru: suppress empty page table walks during aging Message-ID: References: <20260824073806.629593-1-hebaoquan@kylinos.cn> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Stat-Signature: t7asnbbsh7ocsw9ci9d56mipsscjq394 X-Rspamd-Queue-Id: 05929140005 X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1787902981-773590 X-HE-Meta: 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 GcvcsLmr T5X3DCdqHoSHqBmNF4J+6Ja/kWRxA0HBKKzgPDtD119kxYqE5hhWAVannqrJ36+e8/S4OIKDiCIJq2ereB8EJHqQKWaLiq+ZYYMIDJrTqd7QYnTZSZWIaYq+NAV8A9lz6lOsr0RD+uWjZ8a19bG2WS8zYLJgaxcAlOrgseCJcifpSgQF4xTa2y36dq9iwOnoVhwVggENtNl7Z+ARA1Tw5OwaE1qW5tWTVwtkgGGtr/ucl+QzptH/2zVq5lt7/fQ3TumwvMdH8zJMZBE5aWRTHEGluruAVIqcq9mEtZeAUb5URgMA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 08/28/26 at 02:11pm, Barry Song wrote: > On Mon, Aug 24, 2026 at 3:38 PM Baoquan He wrote: > > > > In the current MGLRU, lru_gen_use_mm() will mark an mm for all nodes at > > each context switch, so on each node aging will walk into each mm's page > > table independently. On a multi-NUMA node system, one process launched > > on one or a subset of nodes, its mm is walked by all other nodes's aging > > while finds on pages for their lruvec. This is pure waste (100% walks > > I assume you mean “finds no pages” rather than “find on pages.” You are right, typo, thanks. > > > are empty on those other nodes) > > > > This patch series suppresses these empty page table walks with two > > complementary mechanisms at different levels: > > > > - empty_map (cross-node specifc). At mm granularity, one process's mm > > whose walk found no pages for one node's lruvec is skipped on that > > node between re-scan passes. > > > > - PUD-level Bloom filter (general). One level up from the existing PMD > > filters, it skips any 1GB PUD that had no young entries last > > generation - namely whose 512 PMDs all failed PMD test. Cross-node > > empty walks can prove its effect the best, but it also suppresses > > purely code PUD inside local mms. So it reduces unnecessary walking > > in any workload with cold areas. > > I assume you mean “purely cold PUD” rather than “purely code PUD”. Right, thanks. > > > > > The two complement each other: empty_map reduces the *number* of > > cross-node walks, the PUD-level filter reduces the *cost* of the walks > > that remain. > > I wonder if the PUD-level filter alone would achieve your goal without > requiring the more complex `empty_map` code. > > PUD is already a fairly large granularity, so even if we still do some > redundant work, I wonder if the overhead would be small enough to make > `empty_map` unnecessary? As replied to you in patch 2 thread, will take PUD filter alone in v2 as you suggested, thanks.