Re: [PATCH] sched: fix warning in sched_setaffinity

[Josh Don]

Hi Madadi,

On Sun, Sep 1, 2024 at 7:25 AM Madadi Vineeth Reddy
<vineethr@xxxxxxxxxxxxx> wrote:
>
> 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?

taskset -p 0x2 $$ will affine to cpu 1 :)

I'd recommend using the '-c' arg to specify the mask as a cpulist, as
it is easier to validate.

taskset -c -p 2 $$

>
> Thanks and Regards
> Madadi Vineeth Reddy
>
> > Fixes: 8f9ea86fdf99b ("sched: Always preserve the user requested cpumask")
> > Signed-off-by: Josh Don <joshdon@xxxxxxxxxx>
>

Best,
Josh