Re: [PATCH] sched: fix warning in sched_setaffinity

[Madadi Vineeth Reddy]

Hi Josh Don,

On 04/09/24 03:03, Josh Don wrote:
> 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 for the clarification.

Tested-by: Madadi Vineeth Reddy <vineethr@xxxxxxxxxxxxx>

Thanks and Regards
Madadi Vineeth Reddy

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