[Devel] Re: [RFC][PATCH][1/4] RSS controller setup

[Balbir Singh]

Andrew Morton wrote:
> On Mon, 19 Feb 2007 12:20:26 +0530 Balbir Singh <balbir at in.ibm.com> wrote:
> 
>> This patch sets up the basic controller infrastructure on top of the
>> containers infrastructure. Two files are provided for monitoring
>> and control  memctlr_usage and memctlr_limit.
> 
> The patches use the identifier "memctlr" a lot.  It is hard to remember,
> and unpronounceable.  Something like memcontrol or mem_controller or
> memory_controller would be more typical.
> 

I'll change the name to memory_controller

>> ...
>>
>> +	BUG_ON(!mem);
>> +	if ((buffer = kmalloc(nbytes + 1, GFP_KERNEL)) == 0)
>> +		return -ENOMEM;
> 
> Please prefer to do
> 
> 	buffer = kmalloc(nbytes + 1, GFP_KERNEL);
> 	if (buffer == NULL)
> 		reutrn -ENOMEM;
> 
> ie: avoid the assign-and-test-in-the-same-statement thing.  This affects
> the whole patchset.
> 

I'll fix that

> Also, please don't compare pointers to literal zero like that.  It makes me
> get buried it patches to convert it to "NULL".  I think this is a sparse
> thing.
> 

Good point, I'll fix it.

>> +	buffer[nbytes] = 0;
>> +	if (copy_from_user(buffer, userbuf, nbytes)) {
>> +		ret = -EFAULT;
>> +		goto out_err;
>> +	}
>> +
>> +	container_manage_lock();
>> +	if (container_is_removed(cont)) {
>> +		ret = -ENODEV;
>> +		goto out_unlock;
>> +	}
>> +
>> +	limit = simple_strtoul(buffer, NULL, 10);
>> +	/*
>> +	 * 0 is a valid limit (unlimited resource usage)
>> +	 */
>> +	if (!limit && strcmp(buffer, "0"))
>> +		goto out_unlock;
>> +
>> +	spin_lock(&mem->lock);
>> +	mem->counter.limit = limit;
>> +	spin_unlock(&mem->lock);
> 
> The patches do this a lot: a single atomic assignment with a
> pointless-looking lock/unlock around it.  It's often the case that this
> idiom indicates a bug, or needless locking.  I think the only case where it
> makes sense is when there's some other code somewhere which is doing
> 
> 	spin_lock(&mem->lock);
> 	...
> 	use1(mem->counter.limit);
> 	...
> 	use2(mem->counter.limit);
> 	...
> 	spin_unlock(&mem->lock);
> 
> where use1() and use2() expect the two reads of mem->counter.limit to
> return the same value.
> 
> Is that the case in these patches?  If not, we might have a problem in
> there.
> 

The next set of patches move to atomic values for the limits. That should
fix the locking.

>> +
>> +static ssize_t memctlr_read(struct container *cont, struct cftype *cft,
>> +				struct file *file, char __user *userbuf,
>> +				size_t nbytes, loff_t *ppos)
>> +{
>> +	unsigned long usage, limit;
>> +	char usagebuf[64];		/* Move away from stack later */
>> +	char *s = usagebuf;
>> +	struct memctlr *mem = memctlr_from_cont(cont);
>> +
>> +	spin_lock(&mem->lock);
>> +	usage = mem->counter.usage;
>> +	limit = mem->counter.limit;
>> +	spin_unlock(&mem->lock);
>> +
>> +	s += sprintf(s, "usage %lu, limit %ld\n", usage, limit);
>> +	return simple_read_from_buffer(userbuf, nbytes, ppos, usagebuf,
>> +					s - usagebuf);
>> +}
> 
> This output is hard to parse and to extend.  I'd suggest either two
> separate files, or multi-line output:
> 
> usage: %lu kB
> limit: %lu kB
> 
> and what are the units of these numbers?  Page counts?  If so, please don't
> do that: it requires appplications and humans to know the current kernel's
> page size.
> 

Yes, this looks much better. I'll move to this format. I get myself lost
in "bc" at times, that should have been a hint.

>> +static struct cftype memctlr_usage = {
>> +	.name = "memctlr_usage",
>> +	.read = memctlr_read,
>> +};
>> +
>> +static struct cftype memctlr_limit = {
>> +	.name = "memctlr_limit",
>> +	.write = memctlr_write,
>> +};
>> +
>> +static int memctlr_populate(struct container_subsys *ss,
>> +				struct container *cont)
>> +{
>> +	int rc;
>> +	if ((rc = container_add_file(cont, &memctlr_usage)) < 0)
>> +		return rc;
>> +	if ((rc = container_add_file(cont, &memctlr_limit)) < 0)
> 
> Clean up the first file here?
> 

I used cpuset_populate() as an example to code this one up.
I don't think there is an easy way in containers to clean up
files. I'll double check

>> +		return rc;
>> +	return 0;
>> +}
>> +
>> +static struct container_subsys memctlr_subsys = {
>> +	.name = "memctlr",
>> +	.create = memctlr_create,
>> +	.destroy = memctlr_destroy,
>> +	.populate = memctlr_populate,
>> +};
>> +
>> +int __init memctlr_init(void)
>> +{
>> +	int id;
>> +
>> +	id = container_register_subsys(&memctlr_subsys);
>> +	printk("Initializing memctlr version %s, id %d\n", version, id);
>> +	return id < 0 ? id : 0;
>> +}
>> +
>> +module_init(memctlr_init);
> 


Thanks for the detailed review,

-- 
	Warm Regards,
	Balbir Singh