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b=HIAzlg3ECKZVs6xzOsrw5gyORuG5PWOTZHEB/bfgLKhzYVMoKRXCKO/pDpo8BAjZ3 Jp/0FlG3B69eOYDMVYrFZhj0Xvnagdl4uAFY0sSQRP/HoPgTa6q7anNJ3838q5pfvr jTopbnKKC5OJAtegtZwE+IB3ZvpKWgFP3y/q8fRl2IJXMPRhIGjmwwfZ4Fo4rxdbS+ vLhKFCtF5jTT7DEZLlviC3hnT42AxRe8vaExvYi8G94FZKTdUI2VLBNrN615wfUw76 o9hNxiWzIBfNXXWFSCIgEzxAWh74QdhyAivKRm6HrVgQg0rSlLTbjuQmBqajzzVrel B41emjGw5sOzg== Message-ID: Date: Thu, 27 Aug 2026 19:16:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 00/25] mm/fbatch: drain lru_add_drain() and _all() To: Hugh Dickins , Andrew Morton Cc: Ackerley Tng , Alexander Viro , Baolin Wang , Barry Song , Binbin Wu , Christian Brauner , Christoph Hellwig , Christoph Lameter , Claudio Imbrenda , JP Kobryn , Jan Kara , Jens Axboe , Johannes Weiner , Kairui Song , Kiryl Shutsemau , Lance Yang , Leonardo Bras , Lorenzo Stoakes , Marcelo Tosatti , Matthew Wilcox , Mel Gorman , Miaohe Lin , Michal Hocko , Minchan Kim , Muchun Song , Oscar Salvador , Peter Zijlstra , Qi Zheng , Rik van Riel , Sebastian Andrzej Siewior , Shakeel Butt , Suren Baghdasaryan , Vlastimil Babka , Yang Shi , Yu Zhao , Zach O'Keefe , Zi Yan , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <14a16945-529b-8bc0-ab38-3ea97e54e223@google.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: ww3gghaqnqph3wiyus5ewmwf97dinnc4 X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 245364000A X-Rspam-User: X-HE-Tag: 1787851021-848054 X-HE-Meta: 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 rA6SJy7J YSM3sCoL1hHo9pzmiGt3n02V6Nn5HTtZX+oSTDw6rtpAxcFlQcPjHMktuGxIE3wH0kRA4SMC0EhYpLm+uKIRfhrdB94aKbD3OxWYKiqGTbCpbNuR2cDgTYD8G/LVk6bjlm6DUPlFe0fEy+/uL3cLNf8olGQ1wzVa7fvsaq3zMybglRssjvA9Q4clqJ8i/HtTnpnCjXpiU7DtRK7HkupYjwaEzPPwZzK92URjWgQ26edc10E4JeB/xJOfjDk52FbHCdeLiibGLJkFZdlZJ+ReUwnnUzaK8mIHkIzrcKFPw0xWRlrkvfPxkoLG5vyFJiFeYvmnKV5YWEnikuo28TYlMzekiAhXotNgnthej5b6ASaANgkz4lV/7Uf87k1+sMxAo/S1C Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/24/26 15:49, Hugh Dickins wrote: > This series was prompted by lru_add_drain_all() appearing in watchdog > backtraces: not to blame, but blocked on an unresponsive CPU to run > its workqueue. lru_add_drain_all() is a heavyweight operation, which > we often hope to avoid by a much lighter lru_add_drain(); but even > those local drains can aggravate the lruvec lock contention which > per-cpu fbatches are intended to ease. > > Now let the folios on the lru_add and other per-cpu fbatches remain > there, isolatable, with PG_lru set and refcount unraised, just like > when on an actual LRU. Then many calls to lru_add_drain() and _all() > can be removed. > > It's something I've wanted to do for years, tried several times, but > only hit on a good way to do it a few weeks ago: now it's uncomfortable > to watch others wrestling with the drainage, while I'm sitting on this. > > The key idea came from the "if (!folio_test_clear_lru(folio)) continue;". > If we don't mind missing to take an action on rare occasions, then maybe > we won't mind taking an action on the wrong folio on rare occasions, so > long as it is a folio consenting to PG_lru rules. > > No new locking, but relies on folio_try_get() and folio_test_clear_lru() > even on the lru_add fbatch; with use of bits not set in aligned pointers, > and some try_cmpxchg()ing. Speculative references to folios are already > accepted: this adds another source of them. Sounds very interesting. Thanks for looking into that, LRU caches are a big pain. > > Performance? You (and the bots) tell me. I'm considering this as a > cleanup, to make life easier for developers. I expect that some loads > will show improvement, but also expect some disappointments (perhaps > I go too far against lru_cache_disable()? or not far enough). > > The timing is not so good: middle of a merge window is not a great > time to present new work; but I hope to be taking three weeks off in > two weeks time, so best to get this out in the open early, while I can > respond. I'll be out myself next week. And I'm sure my inbox will be even more of a mess than it is right now when I return. > If it's welcome in principle for 7.4, but too many changes > are demanded, maybe someone else can step up to shepherd it through. I'm certainly interested, my workload is the limiting factor ;) -- Cheers, David