Re: [PATCH 1/3] mm/mempolicy: implement the sysfs-based weighted_interleave interface

[Huang, Ying]

Gregory Price <gregory.price@xxxxxxxxxxxx> writes:

> On Mon, Jan 15, 2024 at 11:18:00AM +0800, Huang, Ying wrote:
>> Gregory Price <gourry.memverge@xxxxxxxxx> writes:
>> 
>> > +static struct iw_table default_iw_table;
>> > +/*
>> > + * iw_table is the sysfs-set interleave weight table, a value of 0
>> > + * denotes that the default_iw_table value should be used.
>> > + *
>> > + * iw_table is RCU protected
>> > + */
>> > +static struct iw_table __rcu *iw_table;
>> > +static DEFINE_MUTEX(iw_table_mtx);
>> 
>> I greped "mtx" in kernel/*.c and mm/*.c and found nothing.  To following
>> the existing coding convention, better to name this as iw_table_mutex or
>> iw_table_lock?
>> 
>
> ack.
>
>> And, I think this is used to protect both iw_table and default_iw_table?
>> If so, it deserves some comments.
>> 
>
> Right now default_iw_table cannot be updated, and so it is neither
> protected nor requires protection.
>
> I planned to add the protection comment in the next patch series, which
> would implement the kernel-side interface for updating the default
> weights during boot/hotplug.
>
> We haven't had the discussion on how/when this should happen yet,
> though, and there's some research to be done.  (i.e. when should DRAM
> weights be set? should the entire table be reweighted on hotplug? etc)

Before that, I'm OK to remove default_iw_table and use hard coded "1" as
default weight for now.

>> > +static ssize_t node_store(struct kobject *kobj, struct kobj_attribute *attr,
>> > +			  const char *buf, size_t count)
>> > +{
>> > +	struct iw_node_attr *node_attr;
>> > +	struct iw_table __rcu *new;
>> > +	struct iw_table __rcu *old;
>> > +	u8 weight = 0;
>> > +
>> > +	node_attr = container_of(attr, struct iw_node_attr, kobj_attr);
>> > +	if (count == 0 || sysfs_streq(buf, ""))
>> > +		weight = 0;
>> > +	else if (kstrtou8(buf, 0, &weight))
>> > +		return -EINVAL;
>> > +
>> > +	new = kmalloc(sizeof(*new), GFP_KERNEL);
>> > +	if (!new)
>> > +		return -ENOMEM;
>> > +
>> > +	mutex_lock(&iw_table_mtx);
>> > +	old = rcu_dereference_protected(iw_table,
>> > +					lockdep_is_held(&iw_table_mtx));
>> > +	/* If value is 0, revert to default weight */
>> > +	weight = weight ? weight : default_iw_table.weights[node_attr->nid];
>> 
>> If we change the default weight in default_iw_table.weights[], how do we
>> identify whether the weight has been customized by users via sysfs?  So,
>> I suggest to use 0 in iw_table for not-customized weight.
>> 
>> And if so, we need to use RCU to access default_iw_table too.
>>
>
> Dumb simplification on my part, I'll walk this back and add the 
>
> if (!weight) weight = default_iw_table[node]
>
> logic back into the allocator paths accordinly.
>
>> > +	memcpy(&new->weights, &old->weights, sizeof(new->weights));
>> > +	new->weights[node_attr->nid] = weight;
>> > +	rcu_assign_pointer(iw_table, new);
>> > +	mutex_unlock(&iw_table_mtx);
>> > +	kfree_rcu(old, rcu);
>> 
>> synchronize_rcu() should be OK here.  It's fast enough in this cold
>> path.  This make it good to define iw_table as
>> 
> I'll take a look.
>
>> u8 __rcu *iw_table;
>> 
>> Then, we only need to allocate nr_node_ids elements now.
>> 
>
> We need nr_possible_nodes to handle hotplug correctly.

nr_node_ids >= num_possible_nodes().  It's larger than any possible node
ID.

> I decided to simplify this down to MAX_NUMNODES *juuuuuust in case*
> "true node hotplug" ever becomes a reality.  If that happens, then
> only allocating space for possible nodes creates a much bigger
> headache on hotplug.
>
> For the sake of that simplification, it seemed better to just eat the
> 1KB.  If you really want me to do that, I will, but the MAX_NUMNODES
> choice was an explicitly defensive choice.

When "true node hotplug" becomes reality, we can make nr_node_ids ==
MAX_NUMNODES.  So, it's safe to use it.  Please take a look at
setup_nr_node_ids().

>> > +static int __init mempolicy_sysfs_init(void)
>> > +{
>> > +	/*
>> > +	 * if sysfs is not enabled MPOL_WEIGHTED_INTERLEAVE defaults to
>> > +	 * MPOL_INTERLEAVE behavior, but is still defined separately to
>> > +	 * allow task-local weighted interleave and system-defaults to
>> > +	 * operate as intended.
>> > +	 *
>> > +	 * In this scenario iw_table cannot (presently) change, so
>> > +	 * there's no need to set up RCU / cleanup code.
>> > +	 */
>> > +	memset(&default_iw_table.weights, 1, sizeof(default_iw_table));
>> 
>> This depends on sizeof(default_iw_table.weights[0]) == 1, I think it's
>> better to use explicit loop here to make the code more robust a little.
>> 
>
> oh hm, you're right.  rookie mistake on my part.
>

--
Best Regards,
Huang, Ying