Re: [PATCH] sched: fix warning in sched_setaffinity

[Madadi Vineeth Reddy <vineethr@linux.ibm.com>]

Hi Josh Don,

On 30/08/24 03:34, Josh Don wrote:
> Commit 8f9ea86fdf99b added some logic to sched_setaffinity that included
> a WARN when a per-task affinity assignment races with a cpuset update.
> 
> Specifically, we can have a race where a cpuset update results in the
> task affinity no longer being a subset of the cpuset. That's fine; we
> have a fallback to instead use the cpuset mask. However, we have a WARN
> set up that will trigger if the cpuset mask has no overlap at all with
> the requested task affinity. This shouldn't be a warning condition; its
> trivial to create this condition.
> 
> Reproduced the warning by the following setup:
> 
> - $PID inside a cpuset cgroup
> - another thread repeatedly switching the cpuset cpus from 1-2 to just 1
> - another thread repeatedly setting the $PID affinity (via taskset) to 2
> 

I was testing the patch using the following two scripts run concurrently:

Script 1:
while true; do
    echo 1 > /sys/fs/cgroup/test_group/cpuset.cpus;
    echo 1-2 > /sys/fs/cgroup/test_group/cpuset.cpus;
done

Script 2:
while true; do
    sudo taskset -p 0x2 $$;
done

However, I am unable to trigger the warning in dmesg on the unpatched kernel.
I was expecting to see the warning as described, but it doesn't seem to appear.

Additionally, I also tried the following script to increase the chances of
triggering the race condition:

while true; do
    echo 1 > /sys/fs/cgroup/test_group/cpuset.cpus;
    sudo taskset -p 0x2 $$;
    sleep 0.1;
    echo 1-2 > /sys/fs/cgroup/test_group/cpuset.cpus;
done

Despite this, the warning still does not appear in dmesg.

Am I missing something in my testing approach, or is there a different setup
required to reproduce the issue?

Thanks and Regards
Madadi Vineeth Reddy

> Fixes: 8f9ea86fdf99b ("sched: Always preserve the user requested cpumask")
> Signed-off-by: Josh Don <joshdon@google.com>