From 5ddf9d53d375e42af49b744bd7c2f8247c6bce15 Mon Sep 17 00:00:00 2001 From: "T.J. Alumbaugh" Date: Wed, 18 Jan 2023 00:18:22 +0000 Subject: [PATCH 13/19] UPSTREAM: mm: multi-gen LRU: section for rmap/PT walk feedback Add a section for lru_gen_look_around() in the code and the design doc. Link: https://lkml.kernel.org/r/20230118001827.1040870-3-talumbau@google.com Change-Id: I5097af63f61b3b69ec2abee6cdbdc33c296df213 Signed-off-by: T.J. Alumbaugh Cc: Yu Zhao Signed-off-by: Andrew Morton (cherry picked from commit db19a43d9b3a8876552f00f656008206ef9a5efa) Bug: 274865848 Signed-off-by: T.J. Mercier --- Documentation/mm/multigen_lru.rst | 14 ++++++++++++++ mm/vmscan.c | 4 ++++ 2 files changed, 18 insertions(+) --- a/Documentation/mm/multigen_lru.rst +++ b/Documentation/mm/multigen_lru.rst @@ -156,6 +156,20 @@ This time-based approach has the followi and memory sizes. 2. It is more reliable because it is directly wired to the OOM killer. +Rmap/PT walk feedback +--------------------- +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. + +``lru_gen_look_around()`` exploits spatial locality to reduce the +trips into the rmap. It scans the adjacent PTEs of a young PTE and +promotes hot pages. If the scan was done cacheline efficiently, it +adds the PMD entry pointing to the PTE table to the Bloom filter. This +forms a feedback loop between the eviction and the aging. + Summary ------- The multi-gen LRU can be disassembled into the following parts: --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -4553,6 +4553,10 @@ static void lru_gen_age_node(struct pgli } } +/****************************************************************************** + * rmap/PT walk feedback + ******************************************************************************/ + /* * This function exploits spatial locality when shrink_folio_list() walks the * rmap. It scans the adjacent PTEs of a young PTE and promotes hot pages. If