linux-kernel - Re: [PATCH 2/7] mm/numa: Introduce nearest_node

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Message-ID: <Z64brsSMAR7cLPUU@gpd3>
Date: Thu, 13 Feb 2025 17:19:58 +0100
From: Andrea Righi <arighi@...dia.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Joel Fernandes <joel@...lfernandes.org>, Ian May <ianm@...dia.com>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] mm/numa: Introduce nearest_node_nodemask()

On Thu, Feb 13, 2025 at 10:57:00AM -0500, Yury Norov wrote:
> On Wed, Feb 12, 2025 at 05:48:09PM +0100, Andrea Righi wrote:
> > Introduce the new helper nearest_node_nodemask() to find the closest
> > node in a specified nodemask from a given starting node.
> > 
> > Returns MAX_NUMNODES if no node is found.
> > 
> > Cc: Yury Norov <yury.norov@...il.com>
> > Signed-off-by: Andrea Righi <arighi@...dia.com>
> 
> Suggested-by: Yury Norov [NVIDIA] <yury.norov@...il.com>

Ok.

> 
> > ---
> >  include/linux/numa.h |  7 +++++++
> >  mm/mempolicy.c       | 32 ++++++++++++++++++++++++++++++++
> >  2 files changed, 39 insertions(+)
> > 
> > diff --git a/include/linux/numa.h b/include/linux/numa.h
> > index 31d8bf8a951a7..e6baaf6051bcf 100644
> > --- a/include/linux/numa.h
> > +++ b/include/linux/numa.h
> > @@ -31,6 +31,8 @@ void __init alloc_offline_node_data(int nid);
> >  /* Generic implementation available */
> >  int numa_nearest_node(int node, unsigned int state);
> >  
> > +int nearest_node_nodemask(int node, nodemask_t *mask);
> > +
> 
> See how you use it. It looks a bit inconsistent to the other functions:
> 
>   #define for_each_node_numadist(node, unvisited)                                \
>          for (int start = (node),                                                \
>               node = nearest_node_nodemask((start), &(unvisited));               \
>               node < MAX_NUMNODES;                                               \
>               node_clear(node, (unvisited)),                                     \
>               node = nearest_node_nodemask((start), &(unvisited)))
>   
> 
> I would suggest to make it aligned with the rest of the API:
> 
>   #define node_clear(node, dst) __node_clear((node), &(dst))
>   static __always_inline void __node_clear(int node, volatile nodemask_t *dstp)
>   {
>           clear_bit(node, dstp->bits);
>   }

Sorry Yury, can you elaborate more on this? What do you mean with
inconsistent, is it the volatile nodemask_t *?

> 
> >  #ifndef memory_add_physaddr_to_nid
> >  int memory_add_physaddr_to_nid(u64 start);
> >  #endif
> > @@ -47,6 +49,11 @@ static inline int numa_nearest_node(int node, unsigned int state)
> >  	return NUMA_NO_NODE;
> >  }
> >  
> > +static inline int nearest_node_nodemask(int node, nodemask_t *mask)
> > +{
> > +	return NUMA_NO_NODE;
> > +}
> > +
> >  static inline int memory_add_physaddr_to_nid(u64 start)
> >  {
> >  	return 0;
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 162407fbf2bc7..1e2acf187ea3a 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -196,6 +196,38 @@ int numa_nearest_node(int node, unsigned int state)
> >  }
> >  EXPORT_SYMBOL_GPL(numa_nearest_node);
> >  
> > +/**
> > + * nearest_node_nodemask - Find the node in @mask at the nearest distance
> > + *			   from @node.
> > + *
> > + * @node: the node to start the search from.
> > + * @mask: a pointer to a nodemask representing the allowed nodes.
> > + *
> > + * This function iterates over all nodes in the given state and calculates
> > + * the distance to the starting node.
> > + *
> > + * Returns the node ID in @mask that is the closest in terms of distance
> > + * from @node, or MAX_NUMNODES if no node is found.
> > + */
> > +int nearest_node_nodemask(int node, nodemask_t *mask)
> > +{
> > +	int dist, n, min_dist = INT_MAX, min_node = MAX_NUMNODES;
> > +
> > +	if (node == NUMA_NO_NODE)
> > +		return MAX_NUMNODES;
> 
> This makes it unclear: you make it legal to pass NUMA_NO_NODE, but
> your function returns something useless. I don't think it would help
> users in any reasonable scenario.
> 
> So, if you don't want user to call this with node == NUMA_NO_NODE,
> just describe it in comment on top of the function. Otherwise, please
> do something useful like 
> 
> 	if (node == NUMA_NO_NODE)
> 		node = current_node;
> 
> I would go with option 1. Notice, node_distance() doesn't bother to
> check against NUMA_NO_NODE.

Hm... is it? Looking at __node_distance(), it doesn't seem really safe to
pass a negative value (maybe I'm missing something?).

Anyway, I'd also prefer to go with option 1 and not implicitly assuming
NUMA_NO_NODE == current node (it feels that it might hide nasty bugs).

So, I can add a comment in the description to clarify that NUMA_NO_NODE is
forbidenx, but what is someone is passing it? Should we WARN_ON_ONCE() at
least?

> 
> > +	for_each_node_mask(n, *mask) {
> > +		dist = node_distance(node, n);
> > +		if (dist < min_dist) {
> > +			min_dist = dist;
> > +			min_node = n;
> > +		}
> > +	}
> > +
> > +	return min_node;
> > +}
> > +EXPORT_SYMBOL_GPL(nearest_node_nodemask);
> > +
> >  struct mempolicy *get_task_policy(struct task_struct *p)
> >  {
> >  	struct mempolicy *pol = p->mempolicy;
> > -- 
> > 2.48.1

Thanks,
-Andrea