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