linux-kernel - Re: [PATCH 2/2] nvme-pci: use blk_mq_max_nr_hw

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Message-ID: <ZKw3H2cJzRPZa29N@kbusch-mbp.dhcp.thefacebook.com>
Date:   Mon, 10 Jul 2023 10:51:43 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        Wen Xiong <wenxiong@...ux.ibm.com>,
        Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, mpe@...erman.id.au,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] nvme-pci: use blk_mq_max_nr_hw_queues() to calculate
 io queues

On Mon, Jul 10, 2023 at 05:14:15PM +0800, 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.
> 
> So Cc our arch(arm & ppc64) & kdump guys wrt. passing 'maxcpus=1' 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.

Is the problem that the managed interrupt sets the effective irq
affinity to an offline CPU? You mentioned observed timeouts; are you
seeing the "completion polled" nvme message?