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

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Date: Wed, 22 May 2024 16:18:00 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: John Meneghini <jmeneghi@...hat.com>, Sagi Grimberg <sagi@...mberg.me>,
        kbusch@...nel.org, hch@....de, 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 20:14, John Meneghini wrote:
> On 5/21/24 06:16, Sagi Grimberg wrote:
>>>>
>>>> Exactly, nvme_mpath_init_ctrl resets the counter.
>>>
>>> Except you're right, the counter reset needs to move to nvme_mpath_init_identify()
>>> or some place that is called on every controller reset.
>>
>> This however raises the question of how much failover/reset tests this patch has seen...
> 
> I has received quite a bit of testing with failover and controller resets.  I shared some of the testing that was done at LSFMM last week.
> 
> It has received enough testing to make me confident that this code is safe.  That is: it won't panic, corrupt data, or otherwise do any harm.  We believe the error paths will not be affected by this change... but I agree that running the error paths could negatively impact the accuracy of the nr_active counters... which could lead to an inaccurate outcome with the queue-depth policy.
> 
> I agree the nr_counter initialize should move to nvme_mpath_init_identify(), or maybe be done there in addition to in nvme_mpath_init_ctrl(). I'm will to make that change now... if that's what people want.  I don't think it would require any extensive retesting.
> 
> /John
> 
> 
I think with Keith's recent proposed patch for fixing io accounting on failover, the 
nvme_mpath_end_request() would be called even for cancelled IO and so the nr_active 
counter shall be adjusted correctly for cancelled IO requests. Having said that, IMO
you shall consider moving initialization of nr_active counter to nvme_mpath_init_identify() 
as that's common function invoked from regular controller initialization code path as well 
the reset code path.

Thanks,
--Nilay