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charset=us-ascii Content-Disposition: inline X-Rspamd-Queue-Id: BDA781C000A X-Stat-Signature: ifco6naj7nhpr4xnr3oas74ybpzk4dwq X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1771606663-486160 X-HE-Meta: U2FsdGVkX1+c95mNoGN+VhwuflCytv/U8qhORPbONazKjA/KMgdgwsex4HsUqeA+l3s93hHGcYd1i7bPO6y7v5y0dV4eCyy2qtc9jfnaKYQut0JoRhp8oOzNSb3zpCTwLF3G/U2QntIyJp96XSk5sPyilrTnromMjyv63hZ4BqTrP5j8OPe8UfhZ3FscJEfwwVJMaaTCTU78UyZpbmhfNPqhFFB4VVXoflFz7wNg5R7t2TtZeLzfS+yEm2TnF8iOk05jg4sSnhQ2AY7BrtipRjEZP1o6uR+D+IXJDb5LMQy/dXeCXIcsjUpd46I+fHcDNICjqL+VftUTZ1oys02HwbHoM+yQFOEAywFscril7wT9vHrofyjl1pvqf6uJUxdMTWIXqfRJOuOrfOR/2WM8zj5V2zm7RnrD+RBuMwitin7Pr/O9GyqzCgYstXcq2MgEn8kh3NfLya1cfZaXpYIjDlwns9Izqxqd/nw7aiK8lGLIBDgfXpSgEs8XeeTXmPTblkIgxhxBZGKs03+LgZ8lonUgWrOw4yLSKqGlCiSmr7Dc2Ad4mjT/w+V9Uxv11l61Mn8Sfg+gU1QQ11qnYFwdNmv8/8HI/1ll6gBlpTLtPFZAA3YpitHZ5ObDpQFHrkrj52rhBDawJZC9CCaoevAxRA6BOCEf+vN5liyiYUcQXSd3gPLp46HNsR87e3S3vIeGkaN2kXIlnkScNVFY+WKKk/ZbAKpnSZZ9ipixPZxZY6Glnnx8gwoYfyG6oQGl8/xm+V+cacgyn1llNQ02xkGU2cpipl7Tb9aVHXvT+vqSuJvotpBfAjIkcXWaBPBUL0xXMnh6+8uFz59VUB7vHXI+TPgPmm+mJ7LI95u/R+HMiSWd2SNQh0nWMImc3wJpkl+LKABnI8BgHMEAI3hreZM3AvMAyNLycL4l9HcSGmXcPRnYfUJ95PA0nAU6QNdYnwsOfC5XMqMEl42qxmPpTmw 2ukXeB/F SHM1wNrZ4tP6mdtXVhKdYiZQDg0LS5+xPnNeOGQ/J8pwof04GsArrmPkEEP+gGUtUOuxU11Dd5wetr3WPUpIOO3qKNRhB63j6/B+wl/pFN7+EE32qpoCu06U/+5GAeYh65x3XX8S3Us4jbW1SawWxK3hQVEOZQz/sW//pYG+qbPdizJZrWSYsVt0w64va23BfQytiTnM7MsFNWCZdL4QKgYk0UVMxqQ6opSLbQ7I79QjZo3WbEac7NcFJg+P8dbxQJ2KRePvBcAWsUGcz5WZvcdZGcf2PJfLGGSetZfQL/X64KD4ZB7zosstlYnocAgX16V1SPxyB9RtlfUqaY6Z/F1T1eFGiNN0lqYL3YBt+qF4R9UhMf+l2tTZYP9pH8JcjAdZkPyDjWk9FpXCTviZJlgxXD4HbtrG5WChBFQj6wiD7jPYqeGWSrxaH9sljjJFIjX06Op8ob4ZKIbLUEIxq8jchYpwnCo7wrO3f 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 Mon, Feb 16, 2026 at 12:00:55PM +0100, Michal Hocko wrote: > On Sat 14-02-26 19:02:19, Leonardo Bras wrote: > > On Wed, Feb 11, 2026 at 05:38:47PM +0100, Michal Hocko wrote: > > > On Wed 11-02-26 09:01:12, Marcelo Tosatti wrote: > > > > On Tue, Feb 10, 2026 at 03:01:10PM +0100, Michal Hocko wrote: > > > [...] > > > > > What about !PREEMPT_RT? We have people running isolated workloads and > > > > > these sorts of pcp disruptions are really unwelcome as well. They do not > > > > > have requirements as strong as RT workloads but the underlying > > > > > fundamental problem is the same. Frederic (now CCed) is working on > > > > > moving those pcp book keeping activities to be executed to the return to > > > > > the userspace which should be taking care of both RT and non-RT > > > > > configurations AFAICS. > > > > > > > > Michal, > > > > > > > > For !PREEMPT_RT, _if_ you select CONFIG_QPW=y, then there is a kernel > > > > boot option qpw=y/n, which controls whether the behaviour will be > > > > similar (the spinlock is taken on local_lock, similar to PREEMPT_RT). > > > > > > My bad. I've misread the config space of this. > > > > > > > If CONFIG_QPW=n, or kernel boot option qpw=n, then only local_lock > > > > (and remote work via work_queue) is used. > > > > > > > > What "pcp book keeping activities" you refer to ? I don't see how > > > > moving certain activities that happen under SLUB or LRU spinlocks > > > > to happen before return to userspace changes things related > > > > to avoidance of CPU interruption ? > > > > > > Essentially delayed operations like pcp state flushing happens on return > > > to the userspace on isolated CPUs. No locking changes are required as > > > the work is still per-cpu. > > > > > > In other words the approach Frederic is working on is to not change the > > > locking of pcp delayed work but instead move that work into well defined > > > place - i.e. return to the userspace. > > > > > > Btw. have you measure the impact of preempt_disbale -> spinlock on hot > > > paths like SLUB sheeves? > > > > Hi Michal, > > > > I have done some study on this (which I presented on Plumbers 2023): > > https://lpc.events/event/17/contributions/1484/ > > > > Since they are per-cpu spinlocks, and the remote operations are not that > > frequent, as per design of the current approach, we are not supposed to see > > contention (I was not able to detect contention even after stress testing > > for weeks), nor relevant cacheline bouncing. > > > > That being said, for RT local_locks already get per-cpu spinlocks, so there > > is only difference for !RT, which as you mention, does preemtp_disable(): > > > > The performance impact noticed was mostly about jumping around in > > executable code, as inlining spinlocks (test #2 on presentation) took care > > of most of the added extra cycles, adding about 4-14 extra cycles per > > lock/unlock cycle. (tested on memcg with kmalloc test) > > > > Yeah, as expected there is some extra cycles, as we are doing extra atomic > > operations (even if in a local cacheline) in !RT case, but this could be > > enabled only if the user thinks this is an ok cost for reducing > > interruptions. > > > > What do you think? > > The fact that the behavior is opt-in for !RT is certainly a plus. I also > do not expect the overhead to be really be really big. To me, a much > more important question is which of the two approaches is easier to > maintain long term. The pcp work needs to be done one way or the other. > Whether we want to tweak locking or do it at a very well defined time is > the bigger question. Without patchset: ================ [ 1188.050725] kmalloc_bench: Avg cycles per kmalloc: 159 With qpw patchset, CONFIG_QPW=n: ================================ [ 50.292190] kmalloc_bench: Avg cycles per kmalloc: 163 With qpw patchset, CONFIG_QPW=y, qpw=0: ======================================= [ 29.872153] kmalloc_bench: Avg cycles per kmalloc: 170 With qpw patchset, CONFIG_QPW=y, qpw=1: ======================================== [ 37.494687] kmalloc_bench: Avg cycles per kmalloc: 190 With PREEMPT_RT enabled, qpw=0: =============================== [ 65.163251] kmalloc_bench: Avg cycles per kmalloc: 181 With PREEMPT_RT enabled, no patchset: ===================================== [ 52.701639] kmalloc_bench: Avg cycles per kmalloc: 185 With PREEMPT_RT enabled, qpw=1: ============================== [ 35.103830] kmalloc_bench: Avg cycles per kmalloc: 196