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From:   Pingfan Liu <piliu@redhat.com>
Date:   Tue, 11 Jul 2023 11:55:15 +0800
Message-ID: <CAF+s44RuqswbosY9kMDx35crviQnxOeuvgNsuE75Bb0Y2Jg2uw@mail.gmail.com>
Subject: Re: [PATCH 2/2] nvme-pci: use blk_mq_max_nr_hw_queues() to calculate
 io queues
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>
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On Mon, Jul 10, 2023 at 5:16=E2=80=AFPM Ming Lei <ming.lei@redhat.com> wrot=
e:
>
> 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=3D1' 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
>

In fact, num_possible_cpus is not broken.

Quote from admin-guide/kernel-parameters.txt
       maxcpus=3D        [SMP] Maximum number of processors that an SMP ker=
nel
                       will bring up during bootup.  maxcpus=3Dn : n >=3D 0=
 limits
                       the kernel to bring up 'n' processors. Surely after
                       bootup you can bring up the other plugged cpu
by executing
                       "echo 1 > /sys/devices/system/cpu/cpuX/online".
So maxcpus
                       only takes effect during system bootup.
                       While n=3D0 is a special case, it is equivalent to "=
nosmp",
                       which also disables the IO APIC.

Here, as it explained, maxcpus only affects the bootup, later, extra
cpus can be online.

> That is documented in Documentation/admin-guide/kdump/kdump.rst.
>
> On arm and ppc64, 'maxcpus=3D1' is passed for kdump kernel, and "maxcpu=
=3D1"

On aarch64 and x86, nr_cpus=3D1 is used, while on ppc64, due to the
implementation, nr_cpus=3D1 can not be supported.


Thanks,

Pingfan

> simply keep one of CPU cores as online, and others as offline.
>
> So Cc our arch(arm & ppc64) & kdump guys wrt. passing 'maxcpus=3D1' for
> kdump kernel.
>
> > setups, otherwise you'll have to paper over it in all kind of
> > drivers.
>
> The issue is only triggered for drivers which use managed irq &
> multiple hw queues.
>
>
> Thanks,
> Ming
>
>
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