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Mon, 23 Feb 2026 19:41:02 +0000 Message-ID: Date: Mon, 23 Feb 2026 14:40:59 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v4 4/4] hazptr: Migrate per-CPU slots to backup slot on context switch To: Boqun Feng Cc: Boqun Feng , Joel Fernandes , "Paul E. McKenney" , linux-kernel@vger.kernel.org, Nicholas Piggin , Michael Ellerman , Greg Kroah-Hartman , Sebastian Andrzej Siewior , Will Deacon , Peter Zijlstra , Alan Stern , John Stultz , Neeraj Upadhyay , Linus Torvalds , Andrew Morton , Frederic Weisbecker , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev References: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com> <20251218014531.3793471-5-mathieu.desnoyers@efficios.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQBPR0101CA0350.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:6b::24) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) MIME-Version: 1.0 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 1NRKLlVo 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 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: List-Subscribe: List-Unsubscribe: On 2026-02-23 11:54, Boqun Feng wrote: > Hi Mathieu, > > On Thu, Dec 18, 2025 at 07:21:28PM -0500, Mathieu Desnoyers wrote: >> On 2025-12-18 17:16, Boqun Feng wrote: >> [...] >>> >>> I would suggest we make CONFIG_PREEMPT_HAZPTR always enabled hence no >>> need for a config, do we have the measurement of the additional cost? >> >> Removing the PREEMPT_HAZPTR brings read-side performance >> from 13.1 down to 5.1 ns. So there is a surprising amount >> of work that goes into list_add/list_del. >> >> I just noticed that I was running a kernel with CONFIG_LIST_HARDENED=y. >> Reruning refscale for PREEMPT_HAZPTR=y with list hardening disabled >> goes from 13.1 ns to 12.4ns, so not a huge win. >> >> I did not notice much difference in terms of scheduler >> performance with a quick hackbench run, but I cannot >> claim it is an extensive benchmark in any way. >> >>> >>> I think you need to add interrupt disabling for chain/unchain because of >>> the potential readers in interrupt and then you can avoid the preempt >>> disabling in hazptr_release() I think. Let's aim for supporting readers >>> in interrupt handler, because at least lockdep needs that. >> >> OK, I'll look into it! >> > > Gentle ping on this. I want to make some forward progress on this ;-) Thanks for the ping, I was focused on other things. I do have an updated branch to post. > > I suggest we make PREEMPT_HAZPTR enabled by default and support readers > in interrupt handler. Yes, that's what I have in my updated branch. > The rest missing part is an async thread, we could > utilize some code in my previous patchset [1]. Let me know whether you > think you have the cycle for this, otherwise I could add this into your > series ;-) Thanks! If that's OK with you, I could post my updated hazptr series for feedback, and leave the async thread adaptation to you. Thanks, Mathieu > > [1]: https://lore.kernel.org/lkml/20250414060055.341516-1-boqun.feng@gmail.com/ > > Regards, > Boqun > >> Thanks, >> >> Mathieu >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. >> https://www.efficios.com -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com