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Mon, 23 Feb 2026 11:54:11 -0500 (EST) Date: Mon, 23 Feb 2026 08:54:10 -0800 From: Boqun Feng To: Mathieu Desnoyers 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 Subject: Re: [RFC PATCH v4 4/4] hazptr: Migrate per-CPU slots to backup slot on context switch Message-ID: References: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com> <20251218014531.3793471-5-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam09 X-Stat-Signature: amgak7p47e6tc5zjeag8sx54x977x1ng X-Rspamd-Queue-Id: 6D67C140005 X-Rspam-User: X-HE-Tag: 1771865655-274986 X-HE-Meta: U2FsdGVkX19q9u8qexDLwvi+m72BVwSlIg50SLG7oqTS/eMKxI2IPHAlsS5z4o7dhjLhphN9bhJB54CLvzLNwGN5G8CaMjD01SNYJxmlmDbmutQiqfzafhkmgUCETbSghDkIhaWVqQBqSPnzFyAV/FrIUvsxs3aIMuL6tKYQIJhDwi5TZURpegxx7+V1ebemOgPV0sKTDd514USTq/3BNMlEqe8k/4gHQH2QI0BOOi5qqX+xHB3c+57VQ1c8Kd2Tu/t3RN5nnanhXLpAhaiQUFtZFXzew+/IcjUgALbpB0CNqwyJJAa+jMjUxgs0oyOztropjyj4zLfaXVRdxAEgsb7ggs8RQMWOFjXaZWZ2GEkfK3jUjiRrfWGXBHXCEsy1U7PfLhsTMP7iBiKtOge/J57LSoIBPjZroLb/AIvL1xGU5sbDeHuQlBZDVEMisOoEIIjmcGrYPNtzAemgMQdHWWEbUB8J4kxadhKJ2mSohZBKZmhgi79l3LLWa9ytxa4vA+IBSL4ll3Z0rY/NpX5otbWhsSauNGQstaUKRVFYvu/auRg0/6m9Ae7AOC2ttyLgQDi2cvDjKlFpMy695pOZiMxXz73cTNkPhBYk0Zwwum/PJb6BUg2IpB4kuxDq4ZTKsnczadQioe3WBQtynekiUFDmaFyPDRS7vtpyB/pHj4I+kM90POzXWsC8D1PD56kCVlGPxCsCsVt9E09PNFyVN5ZVwqambUBioDPrY0gmElfvjGth/BwZ/DMZASbI7fl5FMYa+SMJRwqEbW82DFFN2MCDYRU5Ucc2Pspr7wLFxyfMIvu4nxShdufdb703vN3gERsL9jKDKF9JIAyzEv+Aa2OPv65XpvqyvApyVZKq/GnKJV4tctRvNdSsEy55IUss1SH+Prtj3kTJcgG3ENDHhKppwUWkcA49Qp1M3mnXn21VjEGTZr2Peqy6s72T+M6Iu/52NTYV+BpnWfOVCo2 yisDZ7cT TGUZMx9xV6vS3QS0uNtqS/GihCQmghS31M7/6B9fs1AmVg+1uT5O34X99XT7JUIOCnXL46eQp7yKzDOVpUfRMiTruyd4LLs/6ayIVbrb8ZaX1HCaEUXHl6EoDqmLm87SVIOCnG77HsJEQdoq5KYuIx3KCyW+MH5CUaBRJDTBtOMHV0TnWu70mNPMPksX2k5X+g8plLBF5Qi5/8YC8xBLOfXzuG4XhUQWGcEAzZ72v9zb9OUfGiq2OXg1enK5jCJ4t68QlyegwZ0sDdM36EJ6UnTBYelpq52Lo3AOFzCkeJNgsw32pgQs+d99+k9yuvjU3Cup8ph1oh9lm7mPtWf+OA1E8lbFpC/48GLF4XcHMrqFPCfO3FN2BlMvZ+Ach97WqT6n4ldwKJ+b/cg2TfkTNekEFdlrvHHGo7NSH 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: 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 ;-) I suggest we make PREEMPT_HAZPTR enabled by default and support readers in interrupt handler. 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! [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