Why does processes no longer run on first cpu when cpu affinity is changed by sched_setaffinity() syscall and cpumask contains multiple cpus?

Issue

• Why does processes no longer run on first cpu when cpu affinity is changed by sched_setaffinity() syscall and cpumask contains multiple cpus?

OLD BEHAVIOR

# cat /sys/devices/system/cpu/isolated 
1-3,5-7

# taskset -c 1-3,5-7 sha1sum /dev/zero &
[1] 4116
# taskset -c 1-3,5-7 sha1sum /dev/zero &
[2] 4639
# taskset -c 1-3,5-7 sha1sum /dev/zero &
[3] 4689
# taskset -c 1-3,5-7 sha1sum /dev/zero &
[4] 4716

# ps -o pid,psr,comm -p `pidof sha1sum`
    PID PSR COMMAND
   4116   1 sha1sum
   4639   1 sha1sum
   4689   1 sha1sum
   4716   1 sha1sum

NEW BEHAVIOR

# taskset -c 1-3,5-7 sha1sum /dev/zero &
[1] 5061
# taskset -c 1-3,5-7 sha1sum /dev/zero &
[2] 5062
# taskset -c 1-3,5-7 sha1sum /dev/zero &
[3] 5063
# taskset -c 1-3,5-7 sha1sum /dev/zero &
[4] 5064


# ps -o pid,psr,comm -p `pidof sha1sum`
    PID PSR COMMAND
   5061   5 sha1sum
   5062   6 sha1sum
   5063   7 sha1sum
   5064   1 sha1sum

• In above snippets we can see that with new behavior the tasks are distributed across provided cpuset, whereas with old behavior all the tasks got affinity as first cpu in the cpuset IE cpu#1.