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

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Message-ID: <595e5988-b94d-41ba-a233-f87aed55028d@redhat.com>
Date: Tue, 21 May 2024 09:58:31 -0400
From: John Meneghini <jmeneghi@...hat.com>
To: Hannes Reinecke <hare@...e.de>, 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 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.

/John