linux-kernel - Re: [PATCH v3 1/1] nvme: multipath: Implemented new iopolicy "queue-depth"

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Date: Tue, 21 May 2024 16:23:53 +0200
From: Hannes Reinecke <hare@...e.de>
To: John Meneghini <jmeneghi@...hat.com>, kbusch@...nel.org, hch@....de,
 sagi@...mberg.me, emilne@...hat.com
Cc: linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
 jrani@...estorage.com, randyj@...estorage.com, hare@...nel.org
Subject: Re: [PATCH v3 1/1] nvme: multipath: Implemented new iopolicy
 "queue-depth"

On 5/21/24 15:58, John Meneghini wrote:
> On 5/21/24 02:46, Hannes Reinecke wrote:
>> On 5/20/24 22:20, John Meneghini wrote:
>>> From: "Ewan D. Milne" <emilne@...hat.com>
>>>
> ...
>>> Tested-by: Marco Patalano <mpatalan@...hat.com>
>>> Reviewed-by: Randy Jennings <randyj@...hat.com>
> 
> I need to fix this. Randy doesn't have a redhat.com email address... Cut 
> an paste error :-(
> 
>>> Tested-by: Jyoti Rani <jani@...estorage.com>
>>>
> ...
>>> +void nvme_subsys_iopolicy_update(struct nvme_subsystem *subsys, int 
>>> iopolicy)
>>> +{
>>> +    struct nvme_ctrl *ctrl;
>>> +    int old_iopolicy = READ_ONCE(subsys->iopolicy);
>>> +
>>> +    WRITE_ONCE(subsys->iopolicy, iopolicy);
>>> +
>>> +    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);
>>
>> You always reset the variables here, even if specified iopolicy is
>> the same than the currently active one.
>> I'd rather check if the iopolicy is different before changing the 
>> settings.
> 
> Yes, Keith pointed this out too.  This is actually a feature not a bug.  
> In situations were we want to "reset" the nr_active counters on all 
> controllers the user can simply set the queue-depth iopolicy a second 
> time.  I don't expect users to do this very often... they shouldn't be 
> changing IO policies back and forth too much... but the ability to 
> "reset" the nr_active counters during testing has been very helpful and 
> important to do.  So I'd like to keep this.  Moreover, this is NOT the 
> performance path. I don't see the point in making performance 
> optimizations in a code path that is run once a year.
> 
Please add a comment indicating that.
Just to make it clear that it's by design.

Cheers,

Hannes