linux-kernel - Re: [PATCH v5] nvme: multipath: Implemented new iopolicy "queue-depth"

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Date: Thu, 23 May 2024 11:08:47 -0400
From: John Meneghini <jmeneghi@...hat.com>
To: Christoph Hellwig <hch@....de>
Cc: kbusch@...nel.org, sagi@...mberg.me, emilne@...hat.com,
 linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
 jrani@...estorage.com, randyj@...estorage.com, hare@...nel.org
Subject: Re: [PATCH v5] nvme: multipath: Implemented new iopolicy
 "queue-depth"

On 5/23/24 02:52, Christoph Hellwig wrote:
>> +	/*
>> +	 * queue-depth iopolicy does not need to reference ->current_path
>> +	 * but round-robin needs the last path used to advance to the
>> +	 * next one, and numa will continue to use the last path unless
>> +	 * it is or has become not optimized
>> +	 */
> 
> Can we please turn this into a full sentence?  I.e.:
> 
> 	/*
> 	 * The queue-depth iopolicy does not need to reference ->current_path,
> 	 * but the round-robin iopolicy needs the last path used to advance to
> 	 * the next one, and numa will continue to use the last path unless
> 	 * it is or has become non-optimized.
> 	 */
> 
> ?

I can do that.

>> +	if (iopolicy == NVME_IOPOLICY_QD)
>> +		return nvme_queue_depth_path(head);
>> +
>> +	node = numa_node_id();
>>   	ns = srcu_dereference(head->current_path[node], &head->srcu);
>>   	if (unlikely(!ns))
>>   		return __nvme_find_path(head, node);
>>   
>> -	if (READ_ONCE(head->subsys->iopolicy) == NVME_IOPOLICY_RR)
>> +	if (iopolicy == NVME_IOPOLICY_RR)
>>   		return nvme_round_robin_path(head, node, ns);
>> +
>>   	if (unlikely(!nvme_path_is_optimized(ns)))
>>   		return __nvme_find_path(head, node);
>>   	return ns;
> 
> Also this is growing into the kind of spaghetti code that is on the fast
> path to become unmaintainable.  I'd much rather see the
> srcu_dereference + __nvme_find_path duplicated and have a switch over
> the iopolicies with a separate helper for each of them here than the
> various ifs at different levels.


OK I will turn this into a switch statement.

>> +static void nvme_subsys_iopolicy_update(struct nvme_subsystem *subsys, int iopolicy)
> 
> Overly long line here.

I can fix this.

>> +{
>> +	struct nvme_ctrl *ctrl;
>> +	int old_iopolicy = READ_ONCE(subsys->iopolicy);
>> +
>> +	if (old_iopolicy == iopolicy)
>> +		return;
>> +
>> +	WRITE_ONCE(subsys->iopolicy, iopolicy);
>> +
>> +	/* iopolicy changes reset the counters and clear the mpath by design */
>> +	mutex_lock(&nvme_subsystems_lock);
>> +	list_for_each_entry(ctrl, &subsys->ctrls, subsys_entry) {
>> +		atomic_set(&ctrl->nr_active, 0);
>> +		nvme_mpath_clear_ctrl_paths(ctrl);
>> +	}
>> +	mutex_unlock(&nvme_subsystems_lock);
> 
> You probably want to take the lock over the iopolicy assignment to
> serialize it.  And why do we need the atomic_set here?

Since we are targeting this to 6.11, I will work on refactoring this code.

I'll remove the atomic set here, but I may also add a WARN_ON_ONCE someplace just to be sure our assumptions about the nr_active 
counter state is correct. I agree with Keith that these counters need to be accurate in order for queue-depth to work.

>> +
>> +	pr_notice("%s: changed from %s to %s for subsysnqn %s\n", __func__,
>> +			nvme_iopolicy_names[old_iopolicy], nvme_iopolicy_names[iopolicy],
> 
> Pleae avoid the overly long line here as well.

Hmm... I thought I fixed this already.   Will do.

>>   	NVME_REQ_CANCELLED		= (1 << 0),
>>   	NVME_REQ_USERCMD		= (1 << 1),
>>   	NVME_MPATH_IO_STATS		= (1 << 2),
>> +	NVME_MPATH_CNT_ACTIVE	= (1 << 3),
> 
> This does not match the indentation above.

I must have my tab stop set incorrectly in .vimrc or something.  I'll fix this.

/John