linux-kernel - Re: [PATCH V3 01/14] blk-mq: add blk_mq_max_nr_hw

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Date:   Fri, 11 Aug 2023 13:23:05 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Baoquan He <bhe@...hat.com>, Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-scsi@...r.kernel.org,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Pingfan Liu <piliu@...hat.com>, Dave Young <dyoung@...hat.com>,
        npiggin@...il.com, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        Wen Xiong <wenxiong@...ux.ibm.com>, kexec@...ts.infradead.org,
        Keith Busch <kbusch@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        Christoph Hellwig <hch@....de>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>
Subject: Re: [PATCH V3 01/14] blk-mq: add blk_mq_max_nr_hw_queues()



On 10/08/23 8:31 am, Baoquan He wrote:
> On 08/10/23 at 10:06am, Ming Lei wrote:
>> On Thu, Aug 10, 2023 at 09:18:27AM +0800, Baoquan He wrote:
>>> On 08/10/23 at 08:09am, Ming Lei wrote:
>>>> On Wed, Aug 09, 2023 at 03:44:01PM +0200, Christoph Hellwig wrote:
>>>>> I'm starting to sound like a broken record, but we can't just do random
>>>>> is_kdump checks, and it's not going to get better by resending it again and
>>>>> again.  If kdump kernels limit the number of possible CPUs, it needs to
>>>>> reflected in cpu_possible_map and we need to use that information.
>>>>>
>>>>
>>>> Can you look at previous kdump/arch guys' comment about kdump usage &
>>>> num_possible_cpus?
>>>>
>>>>      https://lore.kernel.org/linux-block/CAF+s44RuqswbosY9kMDx35crviQnxOeuvgNsuE75Bb0Y2Jg2uw@mail.gmail.com/
>>>>      https://lore.kernel.org/linux-block/ZKz912KyFQ7q9qwL@MiWiFi-R3L-srv/
>>>>
>>>> The point is that kdump kernels does not limit the number of possible CPUs.
>>>>
>>>> 1) some archs support 'nr_cpus=1' for kdump kernel, which is fine, since
>>>> num_possible_cpus becomes 1.
>>>
>>> Yes, "nr_cpus=" is strongly suggested in kdump kernel because "nr_cpus="
>>> limits the possible cpu numbers, while "maxcpuss=" only limits the cpu
>>> number which can be brought up during bootup. We noticed this diference
>>> because a large number of possible cpus will cost more memory in kdump
>>> kernel. e.g percpu initialization, even though kdump kernel have set
>>> "maxcpus=1".
>>>
>>> Currently x86 and arm64 all support "nr_cpus=". Pingfan ever spent much
>>> effort to make patches to add "nr_cpus=" support to ppc64, seems ppc64
>>> dev and maintainers do not care about it. Finally the patches are not
>>> accepted, and the work is not continued.
>>>
>>> Now, I am wondering what is the barrier to add "nr_cpus=" to power ach.
>>> Can we reconsider adding 'nr_cpus=' to power arch since real issue
>>> occurred in kdump kernel?
>>
>> If 'nr_cpus=' can be supported on ppc64, this patchset isn't needed.
>>
>>>
>>> As for this patchset, it can be accpeted so that no failure in kdump
>>> kernel is seen on ARCHes w/o "nr_cpus=" support? My personal opinion.
>>
>> IMO 'nr_cpus=' support should be preferred, given it is annoying to
>> maintain two kinds of implementation for kdump kernel from driver
>> viewpoint. I guess kdump things can be simplified too with supporting
>> 'nr_cpus=' only.
> 
> Yes, 'nr_cpus=' is ideal. Not sure if there's some underlying concerns so
> that power people decided to not support it.

Though "nr_cpus=1" is an ideal solution, maintainer was not happy with
the patch as the code changes have impact for regular boot path and
it is likely to cause breakages. So, even if "nr_cpus=1" support for
ppc64 is revived, the change is going to take time to be accepted
upstream.

Also, I see is_kdump_kernel() being used irrespective of "nr_cpus=1"
support for other optimizations in the driver for the special dump
capture environment kdump is.

If there is no other downside for driver code, to use is_kdump_kernel(),
other than the maintainability aspect, I think the above changes are
worth considering.

Thanks
Hari