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Subject: Re: [PATCH] powerpc/pseries: Don't enforce MSI affinity with kdump
To:     Greg Kurz <groug@kaod.org>, Michael Ellerman <mpe@ellerman.id.au>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        =?UTF-8?Q?C=c3=a9dric_Le_Goater?= <clg@kaod.org>,
        stable@vger.kernel.org
References: <20210212164132.821332-1-groug@kaod.org>
From:   Laurent Vivier <lvivier@redhat.com>
Message-ID: <81b4e767-289f-8fb6-9e05-c3fe60beb740@redhat.com>
Date:   Fri, 12 Feb 2021 17:51:57 +0100
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On 12/02/2021 17:41, Greg Kurz wrote:
> Depending on the number of online CPUs in the original kernel, it is
> likely for CPU #0 to be offline in a kdump kernel. The associated IRQs
> in the affinity mappings provided by irq_create_affinity_masks() are
> thus not started by irq_startup(), as per-design with managed IRQs.
> 
> This can be a problem with multi-queue block devices driven by blk-mq :
> such a non-started IRQ is very likely paired with the single queue
> enforced by blk-mq during kdump (see blk_mq_alloc_tag_set()). This
> causes the device to remain silent and likely hangs the guest at
> some point.
> 
> This is a regression caused by commit 9ea69a55b3b9 ("powerpc/pseries:
> Pass MSI affinity to irq_create_mapping()"). Note that this only happens
> with the XIVE interrupt controller because XICS has a workaround to bypass
> affinity, which is activated during kdump with the "noirqdistrib" kernel
> parameter.
> 
> The issue comes from a combination of factors:
> - discrepancy between the number of queues detected by the multi-queue
>   block driver, that was used to create the MSI vectors, and the single
>   queue mode enforced later on by blk-mq because of kdump (i.e. keeping
>   all queues fixes the issue)
> - CPU#0 offline (i.e. kdump always succeed with CPU#0)
> 
> Given that I couldn't reproduce on x86, which seems to always have CPU#0
> online even during kdump, I'm not sure where this should be fixed. Hence
> going for another approach : fine-grained affinity is for performance
> and we don't really care about that during kdump. Simply revert to the
> previous working behavior of ignoring affinity masks in this case only.
> 
> Fixes: 9ea69a55b3b9 ("powerpc/pseries: Pass MSI affinity to irq_create_mapping()")
> Cc: lvivier@redhat.com
> Cc: stable@vger.kernel.org
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  arch/powerpc/platforms/pseries/msi.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/msi.c b/arch/powerpc/platforms/pseries/msi.c
> index b3ac2455faad..29d04b83288d 100644
> --- a/arch/powerpc/platforms/pseries/msi.c
> +++ b/arch/powerpc/platforms/pseries/msi.c
> @@ -458,8 +458,28 @@ static int rtas_setup_msi_irqs(struct pci_dev *pdev, int nvec_in, int type)
>  			return hwirq;
>  		}
>  
> -		virq = irq_create_mapping_affinity(NULL, hwirq,
> -						   entry->affinity);
> +		/*
> +		 * Depending on the number of online CPUs in the original
> +		 * kernel, it is likely for CPU #0 to be offline in a kdump
> +		 * kernel. The associated IRQs in the affinity mappings
> +		 * provided by irq_create_affinity_masks() are thus not
> +		 * started by irq_startup(), as per-design for managed IRQs.
> +		 * This can be a problem with multi-queue block devices driven
> +		 * by blk-mq : such a non-started IRQ is very likely paired
> +		 * with the single queue enforced by blk-mq during kdump (see
> +		 * blk_mq_alloc_tag_set()). This causes the device to remain
> +		 * silent and likely hangs the guest at some point.
> +		 *
> +		 * We don't really care for fine-grained affinity when doing
> +		 * kdump actually : simply ignore the pre-computed affinity
> +		 * masks in this case and let the default mask with all CPUs
> +		 * be used when creating the IRQ mappings.
> +		 */
> +		if (is_kdump_kernel())
> +			virq = irq_create_mapping(NULL, hwirq);
> +		else
> +			virq = irq_create_mapping_affinity(NULL, hwirq,
> +							   entry->affinity);
>  
>  		if (!virq) {
>  			pr_debug("rtas_msi: Failed mapping hwirq %d\n", hwirq);
> 

Reviewed-by: Laurent Vivier <lvivier@redhat.com>