Hello, Michal. > > > > Yes, I do well remember that you are unhappy with this approach. > > Unfortunately, thus far, there is no solution that makes all developers > > happy. You might be glad to hear that we are also looking into other > > solutions, each of which makes some other developers unhappy. So we > > are at least not picking on you alone. :-/ > > No worries I do not feel like a whipping boy here. But do expect me to > argue against the approach. I would also appreciate it if there was some > more information on other attempts, why they have failed. E.g. why > pre-allocation is not an option that works well enough in most > reasonable workloads. Pre-allocating has some drawbacks: a) It is impossible to predict how many pages will be required to cover a demand that is controlled by different workloads on various systems. b) Memory overhead since we do not know how much pages should be preloaded: 100, 200 or 300 As for memory overhead, it is important to reduce it because of embedded devices like phones, where a low memory condition is a big issue. In that sense pre-allocating is something that we strongly would like to avoid. > > I would also appreciate some more thoughts why we > need to optimize for heavy abusers of RCU (like close(open) extremes). > I think here is a small misunderstanding. Please note, that is not only about performance and corner cases. There is a single argument support of the kvfree_rcu(ptr), where maintaining an array in time is needed. The fallback of the single argument case is extrimely slow. Single-argument details is here: https://lkml.org/lkml/2020/4/28/1626 > > > I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@xxxxxxxxxxxxxxxxxxxxxxx > > > that this optimization is not aiming at reasonable workloads. Really, go > > > with pre-allocated buffer and fallback to whatever slow path you have > > > already. Exposing more internals of the allocator is not going to do any > > > good for long term maintainability. > > > > I suggest that you carefully re-read the thread following that email. > > I clearly remember Thomas not being particularly happy that you optimize > for a corner case. I do not remember there being a consensus that this > is the right approach. There was some consensus that this is better than > a gfp flag. Still quite bad though if you ask me. > > > Given a choice between making users unhappy and making developers > > unhappy, I will side with the users each and every time. > > Well, let me rephrase. It is not only about me (as a developer) being > unhappy but also all the side effects this would have for users when > performance of their favorite workload declines for no apparent reason > just because pcp caches are depleted by an unrelated process. > If depleted, we have a special worker that charge it. From the other hand, the pcplist can be depleted by its nature, what _is_ not wrong. But just in case we secure it since you had a concern about it. Could you please specify a real test case or workload you are talking about? Thank you for your comments and help. -- Vlad Rezki
