Received-SPF: pass (google.com: domain of linux-kernel+bounces-186086-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Message-ID: <b4302ee2-584d-48b1-b0fe-77035879c15e@grimberg.me>
Date: Wed, 22 May 2024 13:52:03 +0300
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v3 1/1] nvme: multipath: Implemented new iopolicy
 "queue-depth"
To: Nilay Shroff <nilay@linux.ibm.com>, John Meneghini <jmeneghi@redhat.com>,
 kbusch@kernel.org, hch@lst.de, emilne@redhat.com
Cc: linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
 jrani@purestorage.com, randyj@purestorage.com, hare@kernel.org
References: <20240520202045.427110-1-jmeneghi@redhat.com>
 <20240520202045.427110-2-jmeneghi@redhat.com>
 <945416af-3f8b-40b5-9681-49973beb2cb2@linux.ibm.com>
 <3b8d33db-f2c3-469a-bfa0-8fc52594f243@grimberg.me>
 <95fe3168-ec39-4932-b9fc-26484de49191@linux.ibm.com>
 <da35bc8b-6813-42c8-b446-8a07b08db156@grimberg.me>
 <073ae18a-a80e-4a95-a093-36210a3bb230@grimberg.me>
 <ba2a6973-ba58-492c-9785-31e50ab40957@grimberg.me>
 <8f5ad047-be0b-471c-a708-f2bc464d29ef@redhat.com>
 <b3ea2812-1110-47f6-88f0-0e9c028c350f@linux.ibm.com>
Content-Language: he-IL, en-US
From: Sagi Grimberg <sagi@grimberg.me>
In-Reply-To: <b3ea2812-1110-47f6-88f0-0e9c028c350f@linux.ibm.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit


On 22/05/2024 13:48, Nilay Shroff wrote:
>
> 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.

Yes, and preferably with a comment explaining why its there (despite 
having nothing to do with identify...)