[PATCH -next] cgroup/cpuset: Do not clear xcpus when clearing cpus

[PATCH -next] cgroup/cpuset: Do not clear xcpus when clearing cpus

[Chen Ridong]

After commit 737bb142a00d ("cgroup/cpuset: Make cpuset.cpus.exclusive
independent of cpuset.cpus"), cpuset.cpus.exclusive and cpuset.cpus
became independent. However we found that cpuset.cpus.exclusive.effective
is cleared when cpuset.cpus is clear. To fix this issue, just remove xcpus
clearing when cpuset.cpus is being cleared.

It can be reproduced as below:
cd /sys/fs/cgroup/
mkdir test
echo +cpuset > cgroup.subtree_control
cd test
echo 3 > cpuset.cpus.exclusive
cat cpuset.cpus.exclusive.effective
3
echo > cpuset.cpus
cat cpuset.cpus.exclusive.effective // was cleared

Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 kernel/cgroup/cpuset.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a9b6d56eeffa..248c39bebbe9 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2523,10 +2523,9 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	 * that parsing.  The validate_change() call ensures that cpusets
 	 * with tasks have cpus.
 	 */
-	if (!*buf) {
+	if (!*buf)
 		cpumask_clear(trialcs->cpus_allowed);
-		cpumask_clear(trialcs->effective_xcpus);
-	} else {
+	else {
 		retval = cpulist_parse(buf, trialcs->cpus_allowed);
 		if (retval < 0)
 			return retval;
-- 
2.34.1

Re: [PATCH -next] cgroup/cpuset: Do not clear xcpus when clearing cpus

[Waiman Long]

On 7/31/24 05:21, Chen Ridong wrote:
> After commit 737bb142a00d ("cgroup/cpuset: Make cpuset.cpus.exclusive
> independent of cpuset.cpus"), cpuset.cpus.exclusive and cpuset.cpus
> became independent. However we found that cpuset.cpus.exclusive.effective
> is cleared when cpuset.cpus is clear. To fix this issue, just remove xcpus
> clearing when cpuset.cpus is being cleared.
>
> It can be reproduced as below:
> cd /sys/fs/cgroup/
> mkdir test
> echo +cpuset > cgroup.subtree_control
> cd test
> echo 3 > cpuset.cpus.exclusive
> cat cpuset.cpus.exclusive.effective
> 3
> echo > cpuset.cpus
> cat cpuset.cpus.exclusive.effective // was cleared
>
> Signed-off-by: Chen Ridong <chenridong@huawei.com>
> ---
>   kernel/cgroup/cpuset.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index a9b6d56eeffa..248c39bebbe9 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2523,10 +2523,9 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
>   	 * that parsing.  The validate_change() call ensures that cpusets
>   	 * with tasks have cpus.
>   	 */
> -	if (!*buf) {
> +	if (!*buf)
>   		cpumask_clear(trialcs->cpus_allowed);
> -		cpumask_clear(trialcs->effective_xcpus);
> -	} else {
> +	else {
>   		retval = cpulist_parse(buf, trialcs->cpus_allowed);
>   		if (retval < 0)
>   			return retval;

Yes, that is a corner case bug that has not been properly handled.

Reviewed-by: Waiman Long <longman@redhat.com>

Re: [PATCH -next] cgroup/cpuset: Do not clear xcpus when clearing cpus

[Waiman Long]

On 7/31/24 23:22, Waiman Long wrote:
> On 7/31/24 05:21, Chen Ridong wrote:
>> After commit 737bb142a00d ("cgroup/cpuset: Make cpuset.cpus.exclusive
>> independent of cpuset.cpus"), cpuset.cpus.exclusive and cpuset.cpus
>> became independent. However we found that 
>> cpuset.cpus.exclusive.effective
>> is cleared when cpuset.cpus is clear. To fix this issue, just remove 
>> xcpus
>> clearing when cpuset.cpus is being cleared.
>>
>> It can be reproduced as below:
>> cd /sys/fs/cgroup/
>> mkdir test
>> echo +cpuset > cgroup.subtree_control
>> cd test
>> echo 3 > cpuset.cpus.exclusive
>> cat cpuset.cpus.exclusive.effective
>> 3
>> echo > cpuset.cpus
>> cat cpuset.cpus.exclusive.effective // was cleared
>>
>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>> ---
>>   kernel/cgroup/cpuset.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index a9b6d56eeffa..248c39bebbe9 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -2523,10 +2523,9 @@ static int update_cpumask(struct cpuset *cs, 
>> struct cpuset *trialcs,
>>        * that parsing.  The validate_change() call ensures that cpusets
>>        * with tasks have cpus.
>>        */
>> -    if (!*buf) {
>> +    if (!*buf)
>>           cpumask_clear(trialcs->cpus_allowed);
>> -        cpumask_clear(trialcs->effective_xcpus);
>> -    } else {
>> +    else {
>>           retval = cpulist_parse(buf, trialcs->cpus_allowed);
>>           if (retval < 0)
>>               return retval;
>
> Yes, that is a corner case bug that has not been properly handled.
>
> Reviewed-by: Waiman Long <longman@redhat.com>
>
With a second thought, I think we should keep the clearing of 
effective_xcpus if exclusive_cpus is empty. IOW

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6ba8313f1fc3..2023cd68d9bc 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2516,7 +2516,8 @@ static int update_cpumask(struct cpuset *cs, 
struct cpuset *trialcs,
          */
         if (!*buf) {
                 cpumask_clear(trialcs->cpus_allowed);
-               cpumask_clear(trialcs->effective_xcpus);
+               if (cpumask_empty(trialcs->exclusive_cpus))
+ cpumask_clear(trialcs->effective_xcpus);
         } else {
                 retval = cpulist_parse(buf, trialcs->cpus_allowed);
                 if (retval < 0)

Thanks,
Longman

Re: [PATCH -next] cgroup/cpuset: Do not clear xcpus when clearing cpus

[Saket Kumar Bhaskar]

On Thu, Aug 01, 2024 at 12:31:44PM -0400, Waiman Long wrote:
> 
> On 7/31/24 23:22, Waiman Long wrote:
> > On 7/31/24 05:21, Chen Ridong wrote:
> > > After commit 737bb142a00d ("cgroup/cpuset: Make cpuset.cpus.exclusive
> > > independent of cpuset.cpus"), cpuset.cpus.exclusive and cpuset.cpus
> > > became independent. However we found that
> > > cpuset.cpus.exclusive.effective
> > > is cleared when cpuset.cpus is clear. To fix this issue, just remove
> > > xcpus
> > > clearing when cpuset.cpus is being cleared.
> > > 
> > > It can be reproduced as below:
> > > cd /sys/fs/cgroup/
> > > mkdir test
> > > echo +cpuset > cgroup.subtree_control
> > > cd test
> > > echo 3 > cpuset.cpus.exclusive
> > > cat cpuset.cpus.exclusive.effective
> > > 3
> > > echo > cpuset.cpus
> > > cat cpuset.cpus.exclusive.effective // was cleared
> > > 
> > > Signed-off-by: Chen Ridong <chenridong@huawei.com>
> > > ---
> > >   kernel/cgroup/cpuset.c | 5 ++---
> > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > index a9b6d56eeffa..248c39bebbe9 100644
> > > --- a/kernel/cgroup/cpuset.c
> > > +++ b/kernel/cgroup/cpuset.c
> > > @@ -2523,10 +2523,9 @@ static int update_cpumask(struct cpuset *cs,
> > > struct cpuset *trialcs,
> > >        * that parsing.  The validate_change() call ensures that cpusets
> > >        * with tasks have cpus.
> > >        */
> > > -    if (!*buf) {
> > > +    if (!*buf)
> > >           cpumask_clear(trialcs->cpus_allowed);
> > > -        cpumask_clear(trialcs->effective_xcpus);
> > > -    } else {
> > > +    else {
> > >           retval = cpulist_parse(buf, trialcs->cpus_allowed);
> > >           if (retval < 0)
> > >               return retval;
> > 
> > Yes, that is a corner case bug that has not been properly handled.
> > 
> > Reviewed-by: Waiman Long <longman@redhat.com>
> > 
> With a second thought, I think we should keep the clearing of
> effective_xcpus if exclusive_cpus is empty. IOW
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 6ba8313f1fc3..2023cd68d9bc 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2516,7 +2516,8 @@ static int update_cpumask(struct cpuset *cs, struct
> cpuset *trialcs,
>          */
>         if (!*buf) {
>                 cpumask_clear(trialcs->cpus_allowed);
> -               cpumask_clear(trialcs->effective_xcpus);
> +               if (cpumask_empty(trialcs->exclusive_cpus))
> + cpumask_clear(trialcs->effective_xcpus);
>         } else {
>                 retval = cpulist_parse(buf, trialcs->cpus_allowed);
>                 if (retval < 0)
> 
> Thanks,
> Longman
> 
Hi Longman,

Is there any situation in which we could land here for or after clearing 
exclusive_cpus. AFAIK only way we could landup after clearing exclusive_cpus 
to update_exclusive_cpumask(), which anyway clears effective_xcpus. 
In that case, clearing effective_xcpus would be redundant in update_cpumask().


Also, is there any situation in which we could end up clearing exclusive_cpus
without clearing effective_xcpus as we have a piece of code:

	static inline struct cpumask *fetch_xcpus(struct cpuset *cs)
	{
		return !cpumask_empty(cs->exclusive_cpus) ? cs->exclusive_cpus :
	       	cpumask_empty(cs->effective_xcpus) ? cs->cpus_allowed
						  : cs->effective_xcpus;
	}

Thanks,
Saket