On 19/11/20 16:09, Peter Zijlstra wrote: > On Thu, Nov 19, 2020 at 01:13:20PM +0000, Will Deacon wrote: > >> Sure, but I was talking about what userspace sees, and I don't think it ever >> sees CPUs that have been hotplugged off, right? That is, sched_getaffinity() >> masks its result with the active_mask. > > # for i in /sys/devices/system/cpu/cpu*/online; do echo -n $i ":"; cat $i; done > /sys/devices/system/cpu/cpu1/online :0 > /sys/devices/system/cpu/cpu2/online :1 > /sys/devices/system/cpu/cpu3/online :1 > /sys/devices/system/cpu/cpu4/online :1 > /sys/devices/system/cpu/cpu5/online :1 > /sys/devices/system/cpu/cpu6/online :1 > /sys/devices/system/cpu/cpu7/online :1 > > # grep Cpus_allowed /proc/self/status > Cpus_allowed: ff > Cpus_allowed_list: 0-7 > > > :-) Harumph, so there is that... $ while true; do continue; done & $ PID=$! $ taskset -pc 0-1 $PID pid 849's current affinity list: 0-5 pid 849's new affinity list: 0,1 $ echo 0 > /sys/devices/system/cpu/cpu1/online [12578.545726] CPU1: shutdown [12578.548454] psci: CPU1 killed (polled 0 ms) $ taskset -pc $PID pid 849's current affinity list: 0 $ cat /proc/$PID/status | grep Cpus Cpus_allowed: 03 Cpus_allowed_list: 0-1
