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Date:   Tue, 11 Jul 2023 14:59:35 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
        Wen Xiong <wenxiong@linux.ibm.com>,
        Keith Busch <kbusch@kernel.org>,
        Dave Young <dyoung@redhat.com>, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, mpe@ellerman.id.au,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/2] nvme-pci: use blk_mq_max_nr_hw_queues() to calculate
 io queues
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On 07/11/23 at 11:53am, Ming Lei wrote:
> Hi Baoquan,
> 
> On Tue, Jul 11, 2023 at 11:35:50AM +0800, Baoquan He wrote:
> > On 07/10/23 at 05:14pm, Ming Lei wrote:
> > > On Mon, Jul 10, 2023 at 08:41:09AM +0200, Christoph Hellwig wrote:
> > > > On Sat, Jul 08, 2023 at 10:02:59AM +0800, Ming Lei wrote:
> > > > > Take blk-mq's knowledge into account for calculating io queues.
> > > > > 
> > > > > Fix wrong queue mapping in case of kdump kernel.
> > > > > 
> > > > > On arm and ppc64, 'maxcpus=1' is passed to kdump command line, see
> > > > > `Documentation/admin-guide/kdump/kdump.rst`, so num_possible_cpus()
> > > > > still returns all CPUs.
> > > > 
> > > > That's simply broken.  Please fix the arch code to make sure
> > > > it does not return a bogus num_possible_cpus value for these
> > > 
> > > That is documented in Documentation/admin-guide/kdump/kdump.rst.
> > > 
> > > On arm and ppc64, 'maxcpus=1' is passed for kdump kernel, and "maxcpu=1"
> > > simply keep one of CPU cores as online, and others as offline.
> > 
> > I don't know maxcpus on arm and ppc64 well. But maxcpus=1 or nr_cpus=1
> > are suggested parameter. Because usually nr_cpus=1 is enough to make
> > kdump kernel work well to capture vmcore. However, user is allowed to
> > specify nr_cpus=n (n>1) if they think multiple cpus are needed in kdump
> > kernel. Your hard coding of cpu number in kdump kernel may be not so
> > reasonable.
> 
> As I mentioned, for arm/ppc64, passing 'maxcpus=1' actually follows
> Documentation/admin-guide/kdump/kdump.rst.
> 
> 'nr_cpus=N' just works fine, so not related with this topic.
> 
> After 'maxcpus=1' is passed, kernel only brings up one of cpu cores as
> online during booting, and others still can be put into online by
> userspace. Now this way causes IO timeout on some storage device which
> uses managed irq and supports multiple io queues.
> 
> Here the focus is if passing 'maxcpus=1' is valid for kdump
> kernel, that is we want to hear from our arch/kdump guys.

Yes, 'maxcpus=1' is valid and suggested on ppc64 for kdump kernel
if needed, because there's no 'nr_cpus=' support on ppc64 yet.

> 
> If yes, something needs to be fixed, such as, what this patchset is
> doing.
> 
> > 
> > Please cc kexec mailing list when posting so that people can view the
> > whole thread of discussion.
> 
> Already Cc kexe & arm/powerpc & irq list.
> 
> 
> Thanks,
> Ming
>