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

[Gang Li]

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@xxxxxxxxx>
> > ---
> >   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;

        /* Use the current thread, which saves starting a workqueue 
worker. */
        padata_work_init(&my_work, padata_mt_helper, &ps, 
PADATA_WORK_ONSTACK);
```


> >   
> >   	if (job->size == 0)
> >   		return;
> > @@ -517,7 +517,11 @@ void __init padata_do_multithreaded(struct padata_mt_job *job)
> >   	ps.chunk_size = roundup(ps.chunk_size, job->align);
> >   
> >   	list_for_each_entry(pw, &works, pw_list)
> > -		queue_work(system_unbound_wq, &pw->pw_work);
> > +		if (job->numa_aware)
> > +			queue_work_node((++nid % num_node_state(N_MEMORY)),
> > +					system_unbound_wq, &pw->pw_work);
>
> I think we should use nid = next_node(nid, node_states[N_CPU]) instead of
> ++nid % num_node_state(N_MEMORY).  You are picking the next node with CPU
> to handle the job.
>
> Tim

I agree.