----- On Feb 19, 2020, at 10:52 AM, Tejun Heo tj@xxxxxxxxxx wrote: > On Wed, Feb 19, 2020 at 10:50:35AM -0500, Mathieu Desnoyers wrote: >> I can look into figuring out the commit introducing this issue, which I >> suspect will be close to the introduction of CONFIG_CPUSET into the >> kernel (which was ages ago). I'll check and let you know. > > Oh, yeah, I'm pretty sure it goes way back. I don't think tracking > that down would be necessary. I was just wondering whether it was a > recent change because you said it was a regression. It's most likely not a recent regression, but it has unfortunate effects on the affinity mask which directly affects my ongoing work on the pin_on_cpu() system call [1]. The sched_setaffinity vs cpu hotplug semantic provided by CONFIG_CPUSET=n if fine for the needs on pin_on_cpu(): when a CPU comes back online, those reappear in the affinity mask, but it is not the case with CONFIG_CPUSET=y. I wonder if applying the online cpu masks to the per-thread affinity mask is the correct approach ? I suspect what we may be looking for here is to keep the affinity mask independent of cpu hotplug, and look-up both the per-thread affinity mask and the online cpu mask whenever the scheduler needs to perform "is_cpu_allowed()" to check task placement. Then whenever sched_getaffinity or cpusets try to query the current set of cpus on which a task can run right now, it could also look at both the task's affinity mask and the online cpu mask. Thanks, Mathieu [1] https://lore.kernel.org/r/20200121160312.26545-1-mathieu.desnoyers@xxxxxxxxxxxx -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
