Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity | Date | Thu, 19 Nov 2020 12:47:34 +0000 |
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On 13/11/20 09:37, Will Deacon wrote: > Asymmetric systems may not offer the same level of userspace ISA support > across all CPUs, meaning that some applications cannot be executed by > some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do > not feature support for 32-bit applications on both clusters. > > Although userspace can carefully manage the affinity masks for such > tasks, one place where it is particularly problematic is execve() > because the CPU on which the execve() is occurring may be incompatible > with the new application image. In such a situation, it is desirable to > restrict the affinity mask of the task and ensure that the new image is > entered on a compatible CPU.
> From userspace's point of view, this looks the same as if the > incompatible CPUs have been hotplugged off in its affinity mask.
{pedantic reading warning}
Hotplugged CPUs *can* be set in a task's affinity mask, though interact weirdly with cpusets [1]. Having it be the same as hotplug would mean keeping incompatible CPUs allowed in the affinity mask, but preventing them from being picked via e.g. is_cpu_allowed().
I was actually hoping this could be a feasible approach, i.e. have an extra CPU active mask filter for any task:
cpu_active_mask & arch_cpu_allowed_mask(p)
rather than fiddle with task affinity. Sadly this would also require fixing up pretty much any place that uses cpu_active_mask(), and probably places that use p->cpus_ptr as well. RT / DL balancing comes to mind, because that uses either a task's affinity or a CPU's root domain (which reflects the cpu_active_mask) to figure out where to push a task.
So while I'm wary of hacking up affinity, I fear it might be the lesser evil :(
[1]: https://lore.kernel.org/lkml/1251528473.590671.1579196495905.JavaMail.zimbra@efficios.com/
> In preparation for restricting the affinity mask for compat tasks on > arm64 systems without uniform support for 32-bit applications, introduce > a restrict_cpus_allowed_ptr(), which allows the current affinity mask > for a task to be shrunk to the intersection of a parameter mask. > > Signed-off-by: Will Deacon <will@kernel.org>
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