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Message-ID: <ZNQ64xhCIBU6XM/5@MiWiFi-R3L-srv>
Date: Thu, 10 Aug 2023 09:18:27 +0800
From: Baoquan He <bhe@...hat.com>
To: Ming Lei <ming.lei@...hat.com>, mpe@...erman.id.au,
npiggin@...il.com, christophe.leroy@...roup.eu
Cc: Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
linux-nvme@...ts.infradead.org,
"Martin K . Petersen" <martin.petersen@...cle.com>,
linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
Wen Xiong <wenxiong@...ux.ibm.com>,
Keith Busch <kbusch@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
linux-kernel@...r.kernel.org, Pingfan Liu <piliu@...hat.com>
Subject: Re: [PATCH V3 01/14] blk-mq: add blk_mq_max_nr_hw_queues()
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?
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.
>
> 2) some archs do not support 'nr_cpus=1', and have to rely on
> 'max_cpus=1', so num_possible_cpus isn't changed, and kernel just boots
> with single online cpu. That causes trouble because blk-mq limits single
> queue.
>
> Documentation/admin-guide/kdump/kdump.rst
>
> Thanks,
> Ming
>
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