Messages in this thread | | | Date | Fri, 24 Mar 2023 10:42:09 -0400 | Subject | Re: [PATCH 3/5] cgroup/cpuset: Find another usable CPU if none found in current cpuset | From | Waiman Long <> |
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On 3/24/23 10:32, Will Deacon wrote: > On Fri, Mar 17, 2023 at 10:59:26AM -0400, Waiman Long wrote: >> On 3/17/23 08:27, Michal Koutný wrote: >>> On Tue, Mar 14, 2023 at 04:22:06PM -0400, Waiman Long <longman@redhat.com> wrote: >>>> Some arm64 systems can have asymmetric CPUs where certain tasks are only >>>> runnable on a selected subset of CPUs. >>> Ah, I'm catching up. >>> >>>> This information is not captured in the cpuset. As a result, >>>> task_cpu_possible_mask() may return a mask that have no overlap with >>>> effective_cpus causing new_cpus to become empty. >>> I can see that historically, there was an approach of terminating >>> unaccomodable tasks: >>> 94f9c00f6460 ("arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores") >>> the removal of killing had been made possible with >>> df950811f4a8 ("arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system"). >>> >>> That gives two other alternatives to affinity modification: >>> 2) kill such tasks (not unlike OOM upon memory.max reduction), >>> 3) reject cpuset reduction (violates cgroup v2 delegation). >>> >>> What do you think about 2)? >> Yes, killing it is one possible solution. >> >> (3) doesn't work if the affinity change is due to hot cpu removal. So that >> leaves this patch or (2) as the only alternative. I would like to hear what >> Will and Tejun thinks about it. > The main constraint from the Android side (the lucky ecosystem where these > SoCs tend to show up) is that existing userspace (including 32-bit binaries) > continues to function without modification. So approaches such as killing > tasks or rejecting system calls tend not to work as well, since you > inevitably get divergent behaviour leading to functional breakage rather > than e.g. performance anomalies. > > Having said that, the behaviour we currently have in mainline seems to > be alright, so please don't go out of your way to accomodate these SoCs. > I'm mainly just concerned about introducing any regressions, which is why > I ran my tests on this series
I agree that killing it may be too draconian. I am withholding this patch for now.
Thanks, Longman
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