linux-kernel - Re: [PATCH v2 1/3] mm/mempolicy: implement the sysfs-based weighted

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Date: Mon, 22 Jan 2024 11:58:53 -0500
From: Gregory Price <gregory.price@...verge.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	corbet@....net, akpm@...ux-foundation.org, honggyu.kim@...com,
	rakie.kim@...com, hyeongtak.ji@...com, mhocko@...nel.org,
	vtavarespetr@...ron.com, jgroves@...ron.com,
	ravis.opensrc@...ron.com, sthanneeru@...ron.com,
	emirakhur@...ron.com, Hasan.Maruf@....com, seungjun.ha@...sung.com,
	hannes@...xchg.org, dan.j.williams@...el.com
Subject: Re: [PATCH v2 1/3] mm/mempolicy: implement the sysfs-based
 weighted_interleave interface

On Mon, Jan 22, 2024 at 04:03:53PM +0800, Huang, Ying wrote:
> Gregory Price <gourry.memverge@...il.com> writes:
> 
> > +	/*
> > +	 * The default weight is 1 (for now), when the kernel-internal
> > +	 * default weight array is implemented, this should be updated to
> > +	 * collect the system-default weight of the node if the user passes 0.
> > +	 */
> > +	if (!weight)
> > +		weight = 1;
> 
> From functionality point of view, it's OK to set "weight = 1" here now.
> But when we add system default weight table in the future, we need to
> use "weight = 0".  Otherwise, we cannot distinguish whether the default
> value have been customized via sysfs.  So, I suggest to use that rule.
>
[... snip ...]
> > +	else
> > +		memset(new, 1, nr_node_ids);
> 
> With similar reason as above ("From functionality..."), I suggest to set
> "0" here.
> 

blah - the comment is misleading at best.  The future patch should pass
0 through to the sysfs table and the allocators updated to collect the
system-default weight of the node.

re: doing it this way right now -

I chose to do it this way for now because it ultimately simplifies the
logic in the allocators - all of which will need to be updated with the
future patch set regardless of our implementation choice now.

e.g.

rcu_read_lock();
table = rcu_dereference(iw_table);
if (!policy->wil.cur_weight)
	policy->wil.cur_weight = table ? table[next] : 1;
	                         ^^^ only need single conditional now
rcu_read_unlock();

This logic will need to be updated to use default table values, so I
chose the simpler implementation and left the change to be explicit
at the time the default table is implemented.

If you prefer it the other way now, I can change it, but this seemed
cleaner and simpler for the time being.

> > +	new[node_attr->nid] = weight;
> > +	rcu_assign_pointer(iw_table, new);
> > +	mutex_unlock(&iw_table_lock);
> > +	synchronize_rcu();
> > +	kfree(old);
> > +	return count;
> > +}
> > +
> > +static struct iw_node_attr *node_attrs[MAX_NUMNODES];
> 
> node_attrs[] can be allocated dynamically too.  Just a suggestion.
> 

ack to this and other references to nr_node_ids, will change.

> > +	kfree(old);
> 
> It appears unnecessary to free iw_table in error path.  But this isn't a
> big deal because error path will almost never be executed in practice.
>

checkpatch.pl yells at you if you do null checks before kfree :]

> > +	int err;
> > +	struct kobject *mempolicy_kobj;
> 
> This overrides the global "mempolicy_kobj" defined before function.  But
> I don't think we need the global definition.
> 

Assuming the exit path isn't needed then yeah the global isn't needed.

> > +static int __init mempolicy_sysfs_init(void)
> > +{
> > +	/* A NULL iw_table is interpreted by interleave logic as "all 1s" */
> > +	iw_table = NULL;
> > +	return 0;
> > +}
> > +
> > +static void __exit mempolicy_exit(void) { }
> > +#endif /* CONFIG_SYSFS */
> > +late_initcall(mempolicy_sysfs_init);
> > +module_exit(mempolicy_exit);
> 
> mempolicy.c will not be compiled as module, so we don't need
> module_exit().
> 

ack