On Fri, Jan 24, 2025 at 04:22:19PM +0100, Valentin Schneider wrote: > On 21/01/25 18:00, Uladzislau Rezki wrote: > >> > > >> > As noted before, we defer flushing for vmalloc. We have a lazy-threshold > >> > which can be exposed(if you need it) over sysfs for tuning. So, we can add it. > >> > > >> > >> In a CPU isolation / NOHZ_FULL context, isolated CPUs will be running a > >> single userspace application that will never enter the kernel, unless > >> forced to by some interference (e.g. IPI sent from a housekeeping CPU). > >> > >> Increasing the lazy threshold would unfortunately only delay the > >> interference - housekeeping CPUs are free to run whatever, and so they will > >> eventually cause the lazy threshold to be hit and IPI all the CPUs, > >> including the isolated/NOHZ_FULL ones. > >> > > Do you have any testing results for your workload? I mean how much > > potentially we can allocate. Again, maybe it is just enough to back > > and once per-hour offload it. > > > > Potentially as much as you want... In our Openshift environments, you can > get any sort of container executing on the housekeeping CPUs and they're > free to do pretty much whatever they want. Per CPU isolation they're not > allowed/meant to disturb isolated CPUs, however. > > > Apart of that how critical IPIing CPUs affect your workloads? > > > > If I'm being pedantic, a single IPI to an isolated CPU breaks the > isolation. If we can't quiesce IPIs to isolated CPUs, then we can't > guarantee that whatever is running on the isolated CPUs is actually > isolated / shielded from third party interference. > I see. I thought you are fixing some issue. I do not see a straight forward way how to remove such "distortion". Probably we can block the range which we defer for flushing. But it also can be problematic because of other constraints. Thanks! -- Uladzislau Rezki
