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charset=us-ascii Content-Disposition: inline X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: EBA961C0006 X-Stat-Signature: y753zp16czge4sm8k4jfgo6j3jot84b7 X-Rspam-User: X-HE-Tag: 1770824590-839426 X-HE-Meta: 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 YW8Yelgq rRGrGFH3Mh0+Ap1J9SO1gaQ6CQFrudm4y3jiaFoz9Z3+IyHpUocuuQvf9B9A4wYpHZdOysQRKKSj4o+Vd9zZXEqySN/IZVnqaGRfO9dqDv5j9W2YINF1d2KGe7RPCle81aWvQNaRo2kTqRRY/g/tb/96Gqs4SU3AC13FDvJyQp2b1AtD1fDzWCdETpc1XGyDZe0KaNc0hc+dnc7+OqkNT059UKMhyC+nIDZ6Go8ndyG/MtWQJKHKNCBKCGTeZH2YH/NdfAaEBhz2C+MXaZc+fDmmAiHc15WsYwYxSkjKBLFVLOytCXVwLFTutCML1cEE/Qqe3xO9ISiMHAUC7/NilNjs/zlseCHtGiWGp/3cRH8+etBz9UJcV0D5YdulCjoBuSDcRoMvXK9rMw+k= 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 Tue, Feb 10, 2026 at 03:01:10PM +0100, Michal Hocko wrote: > On Fri 06-02-26 11:34:30, Marcelo Tosatti wrote: > > The problem: > > Some places in the kernel implement a parallel programming strategy > > consisting on local_locks() for most of the work, and some rare remote > > operations are scheduled on target cpu. This keeps cache bouncing low since > > cacheline tends to be mostly local, and avoids the cost of locks in non-RT > > kernels, even though the very few remote operations will be expensive due > > to scheduling overhead. > > > > On the other hand, for RT workloads this can represent a problem: getting > > an important workload scheduled out to deal with remote requests is > > sure to introduce unexpected deadline misses. > > > > The idea: > > Currently with PREEMPT_RT=y, local_locks() become per-cpu spinlocks. > > In this case, instead of scheduling work on a remote cpu, it should > > be safe to grab that remote cpu's per-cpu spinlock and run the required > > work locally. That major cost, which is un/locking in every local function, > > already happens in PREEMPT_RT. > > > > Also, there is no need to worry about extra cache bouncing: > > The cacheline invalidation already happens due to schedule_work_on(). > > > > This will avoid schedule_work_on(), and thus avoid scheduling-out an > > RT workload. > > > > Proposed solution: > > A new interface called Queue PerCPU Work (QPW), which should replace > > Work Queue in the above mentioned use case. > > > > If PREEMPT_RT=n this interfaces just wraps the current > > local_locks + WorkQueue behavior, so no expected change in runtime. > > > > If PREEMPT_RT=y, or CONFIG_QPW=y, queue_percpu_work_on(cpu,...) will > > lock that cpu's per-cpu structure and perform work on it locally. > > This is possible because on functions that can be used for performing > > remote work on remote per-cpu structures, the local_lock (which is already > > a this_cpu spinlock()), will be replaced by a qpw_spinlock(), which > > is able to get the per_cpu spinlock() for the cpu passed as parameter. > > 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). 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 ? Thanks