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Message-ID: <cc135e4ba87bf64b384a529ccbd4c644bb135266.camel@linux.intel.com>
Date: Wed, 17 Jan 2024 14:14:56 -0800
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Gang Li <gang.li@...ux.dev>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, Mike
Kravetz <mike.kravetz@...cle.com>, David Rientjes <rientjes@...gle.com>,
linux-kernel@...r.kernel.org, ligang.bdlg@...edance.com, David Hildenbrand
<david@...hat.com>, Muchun Song <muchun.song@...ux.dev>
Subject: Re: [PATCH v3 3/7] padata: dispatch works on different nodes
On Mon, 2024-01-15 at 16:57 +0800, Gang Li wrote:
>
> 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@...ux.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);
>
> ```
>
I am thinking something like
static volatile atomic_t last_used_nid;
list_for_each_entry(pw, &works, pw_list)
if (job->numa_aware) {
int old_node = atomic_read(&last_used_nid);
do {
nid = next_node_in(old_node, node_states[N_CPU]);
} while (!atomic_try_cmpxchg(&last_used_nid, &old_node, nid));
queue_work_node(nid, system_unbound_wq, &pw->pw_work);
} else {
queue_work(system_unbound_wq, &pw->pw_work);
}
Note that we need to use next_node_in so we'll wrap around the node mask.
Tim
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