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

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Message-ID: <ZKyxXgJXk0XPBxe2@ovpn-8-26.pek2.redhat.com>
Date:   Tue, 11 Jul 2023 09:33:18 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Keith Busch <kbusch@...nel.org>
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>, ming.lei@...hat.com
Subject: Re: [PATCH 2/2] nvme-pci: use blk_mq_max_nr_hw_queues() to calculate
 io queues

On Mon, Jul 10, 2023 at 10:51:43AM -0600, Keith Busch wrote:
> 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

Yes, the problem is that blk-mq only creates hctx0, so nvme-pci
translate it into hctx0's nvme_queue, this way is actually wrong, cause
blk-mq's view on queue topo isn't same with nvme's view.

> seeing the "completion polled" nvme message?

Yes, "completion polled" can be observed. Meantime the warning in
__irq_startup_managed() can be triggered from
nvme_timeout()->nvme_poll_irqdisable()->enable_irq().


Thanks,
Ming