Re: [PATCH v2] sched/numa: Add statistics of numa balance task migration and swap

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Libo,

On 4/29/2025 7:10 AM, Libo Chen wrote:
> Hi Chen Yu,
> 
> I think this is quite useful! I hope it can be picked up.
> 
> I have one comment below
> 
> On 4/8/25 03:14, Chen Yu wrote:
>> On systems with NUMA balancing enabled, it is found that tracking
>> the task activities due to NUMA balancing is helpful. NUMA balancing
>> has two mechanisms for task migration: one is to migrate the task to
>> an idle CPU in its preferred node, the other is to swap tasks on
>> different nodes if they are on each other's preferred node.
>>
>> The kernel already has NUMA page migration statistics in
>> /sys/fs/cgroup/mytest/memory.stat and /proc/{PID}/sched,
>> but does not have statistics for task migration/swap.
>> Add the task migration and swap count accordingly.
>>
>> The following two new fields:
>>
>> numa_task_migrated
>> numa_task_swapped
>>
>> will be displayed in both
>> /sys/fs/cgroup/{GROUP}/memory.stat and /proc/{PID}/sched
>>
>> Introducing both pertask and permemcg NUMA balancing statistics helps
>> to quickly evaluate the performance and resource usage of the target
>> workload. For example, the user can first identify the container which
>> has high NUMA balance activity and then narrow down to a specific task
>> within that group, and tune the memory policy of that task.
>> In summary, it is plausible to iterate the /proc/$pid/sched to find the
>> offending task, but the introduction of per memcg tasks' Numa balancing
>> aggregated  activity can further help users identify the task in a
>> divide-and-conquer way.
>>
>> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
>> Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
>> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
>> ---
>> v1->v2:
>> Update the Documentation/admin-guide/cgroup-v2.rst. (Michal)
>> ---
>>   Documentation/admin-guide/cgroup-v2.rst |  6 ++++++
>>   include/linux/sched.h                   |  4 ++++
>>   include/linux/vm_event_item.h           |  2 ++
>>   kernel/sched/core.c                     | 10 ++++++++--
>>   kernel/sched/debug.c                    |  4 ++++
>>   mm/memcontrol.c                         |  2 ++
>>   mm/vmstat.c                             |  2 ++
>>   7 files changed, 28 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
>> index f293a13b42ed..b698be14942c 100644
>> --- a/Documentation/admin-guide/cgroup-v2.rst
>> +++ b/Documentation/admin-guide/cgroup-v2.rst
>> @@ -1652,6 +1652,12 @@ The following nested keys are defined.
>>   	  numa_hint_faults (npn)
>>   		Number of NUMA hinting faults.
>>   
>> +	  numa_task_migrated (npn)
>> +		Number of task migration by NUMA balancing.
>> +
>> +	  numa_task_swapped (npn)
>> +		Number of task swap by NUMA balancing.
>> +
>>   	  pgdemote_kswapd
>>   		Number of pages demoted by kswapd.
>>   
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 56ddeb37b5cd..2e91326c16ec 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -549,6 +549,10 @@ struct sched_statistics {
>>   	u64				nr_failed_migrations_running;
>>   	u64				nr_failed_migrations_hot;
>>   	u64				nr_forced_migrations;
>> +#ifdef CONFIG_NUMA_BALANCING
>> +	u64				numa_task_migrated;
>> +	u64				numa_task_swapped;
>> +#endif
>>   
>>   	u64				nr_wakeups;
>>   	u64				nr_wakeups_sync;
>> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
>> index 5a37cb2b6f93..df8a1b30930f 100644
>> --- a/include/linux/vm_event_item.h
>> +++ b/include/linux/vm_event_item.h
>> @@ -64,6 +64,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>>   		NUMA_HINT_FAULTS,
>>   		NUMA_HINT_FAULTS_LOCAL,
>>   		NUMA_PAGE_MIGRATE,
>> +		NUMA_TASK_MIGRATE,
>> +		NUMA_TASK_SWAP,
>>   #endif
>>   #ifdef CONFIG_MIGRATION
>>   		PGMIGRATE_SUCCESS, PGMIGRATE_FAIL,
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index b434c2f7e3c1..54e7d63f7785 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3352,6 +3352,11 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>>   #ifdef CONFIG_NUMA_BALANCING
>>   static void __migrate_swap_task(struct task_struct *p, int cpu)
>>   {
>> +	__schedstat_inc(p->stats.numa_task_swapped);
>> +
>> +	if (p->mm)
>> +		count_memcg_events_mm(p->mm, NUMA_TASK_SWAP, 1);
>> +
> 
> Is p->mm check necessary? I am pretty sure a !p->mm task cannot reach to this point,
> task_tick_numa() will filter out those tasks, no hinting page fault on such ones.

Right, kernel thread is not supposed to reach here.

The swap task is obtained from env->best_task, which is assigned by the 
functions task_numa_compare() and task_numa_assign(), iff that task has
a valid numa_preferred_nid . In other words, only when the swap task is 
a non-kernel thread will it possess a valid numa_preferred_nid.

Let me remove these checks both in __migrate_swap_task and 
migrate_task_to().

Thanks,
Chenyu

> We can add a likely() macro here to minimize the overhead if there is a reason to
> keep that check.
> 
> Same comment to the other one in migrate_task_to().
> 
>