Re: [PATCH v3 3/7] padata: dispatch works on different nodes

[Gang Li <gang.li@linux.dev>]

On 2024/1/13 02:27, Tim Chen wrote:
> On Fri, 2024-01-12 at 15:09 +0800, Gang Li wrote:
>> On 2024/1/12 01:50, Tim Chen wrote:
>>> On Tue, 2024-01-02 at 21:12 +0800, Gang Li wrote:
>>>> When a group of tasks that access different nodes are scheduled on the
>>>> same node, they may encounter bandwidth bottlenecks and access latency.
>>>>
>>>> Thus, numa_aware flag is introduced here, allowing tasks to be
>>>> distributed across different nodes to fully utilize the advantage of
>>>> multi-node systems.
>>>>
>>>> Signed-off-by: Gang Li <gang.li@linux.dev>
>>>> ---
>>>>    include/linux/padata.h | 3 +++
>>>>    kernel/padata.c        | 8 ++++++--
>>>>    mm/mm_init.c           | 1 +
>>>>    3 files changed, 10 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/linux/padata.h b/include/linux/padata.h
>>>> index 495b16b6b4d72..f79ccd50e7f40 100644
>>>> --- a/include/linux/padata.h
>>>> +++ b/include/linux/padata.h
>>>> @@ -137,6 +137,8 @@ struct padata_shell {
>>>>     *             appropriate for one worker thread to do at once.
>>>>     * @max_threads: Max threads to use for the job, actual number may be less
>>>>     *               depending on task size and minimum chunk size.
>>>> + * @numa_aware: Dispatch jobs to different nodes. If a node only has memory but
>>>> + *              no CPU, dispatch its jobs to a random CPU.
>>>>     */
>>>>    struct padata_mt_job {
>>>>    	void (*thread_fn)(unsigned long start, unsigned long end, void *arg);
>>>> @@ -146,6 +148,7 @@ struct padata_mt_job {
>>>>    	unsigned long		align;
>>>>    	unsigned long		min_chunk;
>>>>    	int			max_threads;
>>>> +	bool			numa_aware;
>>>>    };
>>>>    
>>>>    /**
>>>> diff --git a/kernel/padata.c b/kernel/padata.c
>>>> index 179fb1518070c..1c2b3a337479e 100644
>>>> --- a/kernel/padata.c
>>>> +++ b/kernel/padata.c
>>>> @@ -485,7 +485,7 @@ void __init padata_do_multithreaded(struct padata_mt_job *job)
>>>>    	struct padata_work my_work, *pw;
>>>>    	struct padata_mt_job_state ps;
>>>>    	LIST_HEAD(works);
>>>> -	int nworks;
>>>> +	int nworks, nid = 0;
>>>
>>> If we always start from 0, we may be biased towards the low numbered node,
>>> and not use high numbered nodes at all.  Suggest you do
>>> static nid = 0;
>>>
>>
>> When we use `static`, if there are multiple parallel calls to
>> `padata_do_multithreaded`, it may result in an uneven distribution of
>> tasks for each padata_do_multithreaded.
>>
>> We can make the following modifications to address this issue.
>>
>> ```
>> diff --git a/kernel/padata.c b/kernel/padata.c
>> index 1c2b3a337479e..925e48df6dd8d 100644
>> --- a/kernel/padata.c
>> +++ b/kernel/padata.c
>> @@ -485,7 +485,8 @@ void __init padata_do_multithreaded(struct
>> padata_mt_job *job)
>>           struct padata_work my_work, *pw;
>>           struct padata_mt_job_state ps;
>>           LIST_HEAD(works);
>> -       int nworks, nid = 0;
>> +       int nworks, nid;
>> +       static volatile int global_nid = 0;
>>
>>           if (job->size == 0)
>>                   return;
>> @@ -516,12 +517,15 @@ void __init padata_do_multithreaded(struct
>> padata_mt_job *job)
>>           ps.chunk_size = max(ps.chunk_size, job->min_chunk);
>>           ps.chunk_size = roundup(ps.chunk_size, job->align);
>>
>> +       nid = global_nid;
>>           list_for_each_entry(pw, &works, pw_list)
>> -               if (job->numa_aware)
>> -                       queue_work_node((++nid % num_node_state(N_MEMORY)),
>> -                                       system_unbound_wq, &pw->pw_work);
>> -               else
>> +               if (job->numa_aware) {
>> +                       queue_work_node(nid, system_unbound_wq,
>> &pw->pw_work);
>> +                       nid = next_node(nid, node_states[N_CPU]);
>> +               } else
>>                           queue_work(system_unbound_wq, &pw->pw_work);
>> +       if (job->numa_aware)
>> +               global_nid = nid;
> 
> Thinking more about it, there could still be multiple threads working
> at the same time with stale global_nid.  We should probably do a compare
> exchange of global_nid with new nid only if the global nid was unchanged.
> Otherwise we should go to the next node with the changed global nid before
> we queue the job.
> 
> Tim
> 
How about:
```
nid = global_nid;
list_for_each_entry(pw, &works, pw_list)
	if (job->numa_aware) {
		int old_node = nid;
		queue_work_node(nid, system_unbound_wq, &pw->pw_work);
		nid = next_node(nid, node_states[N_CPU]);
		cmpxchg(&global_nid, old_node, nid);
	} else
		queue_work(system_unbound_wq, &pw->pw_work);

```