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[24.6.216.183]) by smtp.gmail.com with ESMTPSA id o11sm4479116pfu.150.2022.01.09.11.34.41 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Sun, 09 Jan 2022 11:34:42 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.120.0.1.13\)) Subject: Re: [PATCH 16/23] sched: Use lightweight hazard pointers to grab lazy mms From: Nadav Amit In-Reply-To: Date: Sun, 9 Jan 2022 11:34:40 -0800 Cc: Linus Torvalds , Andy Lutomirski , Andrew Morton , Linux-MM , Nicholas Piggin , Anton Blanchard , Benjamin Herrenschmidt , Paul Mackerras , Randy Dunlap , linux-arch , X86 ML , Dave Hansen , Peter Zijlstra , Mathieu Desnoyers Content-Transfer-Encoding: 7bit Message-Id: <1B6896F0-7A51-4936-8B50-0B86551FA3B7@gmail.com> References: <7c9c388c388df8e88bb5d14828053ac0cb11cf69.1641659630.git.luto@kernel.org> <739A3109-04DD-4BA5-A02B-52EE30E820AE@gmail.com> To: Rik van Riel X-Mailer: Apple Mail (2.3654.120.0.1.13) Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=n3fjHsp+; spf=pass (imf08.hostedemail.com: domain of nadav.amit@gmail.com designates 209.85.215.182 as permitted sender) smtp.mailfrom=nadav.amit@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspamd-Queue-Id: F20BE16000F X-Stat-Signature: 5s5ynqyjitz3fii73nzg1krnynezebpe X-Rspamd-Server: rspam04 X-HE-Tag: 1641756884-481915 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 Jan 9, 2022, at 11:22 AM, Rik van Riel wrote: > > On Sun, 2022-01-09 at 00:49 -0800, Nadav Amit wrote: >> >> It is possible for instance to get rid of is_lazy, keep the CPU >> on mm_cpumask when switching to kernel thread, and then if/when >> an IPI is received remove it from cpumask to avoid further >> unnecessary TLB shootdown IPIs. >> >> I do not know whether it is a pure win, but there is a tradeoff. > > That's not a win at all. It is what we used to have before > the lazy mm stuff was re-introduced, and it led to quite a > few unnecessary IPIs, which are especially slow on virtual > machines, where the target CPU may not be running. You make a good point about VMs. IIUC Lazy-TLB serves several goals: 1. Avoid arch address-space switch to save switching time and TLB misses. 2. Prevent unnecessary IPIs while kernel threads run. 3. Avoid cache-contention on mm_cpumask when switching to a kernel thread. Your concern is with (2), and this one is easy to keep regardless of the rest. I am not sure that (3) is actually helpful, since it might lead to more cache activity than without lazy-TLB, but that is somewhat orthogonal to everything else. As for (1), which is the most fragile aspect, unless you use shadow page-tables, I am not sure there is a significant benefit.