linux-kernel - Re: [PATCH 2/2] blk-mq: simplify queue mapping & schedule with each possisble CPU

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Date:   Wed, 17 Jan 2018 11:07:48 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Ming Lei <ming.lei@...hat.com>,
        "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     linux-block@...r.kernel.org, Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Christoph Hellwig <hch@...radead.org>,
        Stefan Haberland <sth@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        James Smart <james.smart@...adcom.com>,
        Jens Axboe <axboe@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 2/2] blk-mq: simplify queue mapping & schedule with each
 possisble CPU



On 01/17/2018 10:57 AM, Ming Lei wrote:
> Hi Jianchao,
> 
> On Wed, Jan 17, 2018 at 04:09:11PM +0800, jianchao.wang wrote:
>> Hi ming 
>>
>> Thanks for your kindly response.
>>
>> On 01/17/2018 02:22 PM, Ming Lei wrote:
>>> This warning can't be removed completely, for example, the CPU figured
>>> in blk_mq_hctx_next_cpu(hctx) can be put on again just after the
>>> following call returns and before __blk_mq_run_hw_queue() is scheduled
>>> to run.
>>>
>>> 	kblockd_mod_delayed_work_on(blk_mq_hctx_next_cpu(hctx), &hctx->run_work, msecs_to_jiffies(msecs))
>> We could use cpu_active in __blk_mq_run_hw_queue() to narrow the window.
>> There is a big gap between cpu_online and cpu_active. rebind_workers is also between them.
> 
> This warning is harmless, also you can't reproduce it without help of your
> special patch, I guess, :-) So the window shouldn't be a big deal. 

FWIW, every WARN_ON is problematic since there are people running with panic_on_warn.
If a condition can happen we should not use WARN_ON but something else.