Date:   Tue, 15 Jun 2021 14:57:07 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ming Lei <ming.lei@...hat.com>, Christoph Hellwig <hch@....de>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Sagi Grimberg <sagi@...mberg.me>,
        linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
        Keith Busch <keith.busch@...el.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Sumit Saxena <sumit.saxena@...adcom.com>,
        Kashyap Desai <kashyap.desai@...adcom.com>,
        Shivasharan Srikanteshwara 
        <shivasharan.srikanteshwara@...adcom.com>
Subject: Re: [patch v6 3/7] genirq/affinity: Add new callback for
 (re)calculating interrupt sets

On Sat, Feb 16, 2019 at 06:13:09PM +0100, Thomas Gleixner wrote:
> From: Ming Lei <ming.lei@...hat.com>
> 
> The interrupt affinity spreading mechanism supports to spread out
> affinities for one or more interrupt sets. A interrupt set contains one or
> more interrupts. Each set is mapped to a specific functionality of a
> device, e.g. general I/O queues and read I/O queus of multiqueue block
> devices.
> 
> The number of interrupts per set is defined by the driver. It depends on
> the total number of available interrupts for the device, which is
> determined by the PCI capabilites and the availability of underlying CPU
> resources, and the number of queues which the device provides and the
> driver wants to instantiate.
> 
> The driver passes initial configuration for the interrupt allocation via a
> pointer to struct irq_affinity.
> 
> Right now the allocation mechanism is complex as it requires to have a loop
> in the driver to determine the maximum number of interrupts which are
> provided by the PCI capabilities and the underlying CPU resources.  This
> loop would have to be replicated in every driver which wants to utilize
> this mechanism. That's unwanted code duplication and error prone.
> 
> In order to move this into generic facilities it is required to have a
> mechanism, which allows the recalculation of the interrupt sets and their
> size, in the core code. As the core code does not have any knowledge about the
> underlying device, a driver specific callback is required in struct
> irq_affinity, which can be invoked by the core code. The callback gets the
> number of available interupts as an argument, so the driver can calculate the
> corresponding number and size of interrupt sets.
> 
> At the moment the struct irq_affinity pointer which is handed in from the
> driver and passed through to several core functions is marked 'const', but for
> the callback to be able to modify the data in the struct it's required to
> remove the 'const' qualifier.
> 
> Add the optional callback to struct irq_affinity, which allows drivers to
> recalculate the number and size of interrupt sets and remove the 'const'
> qualifier.
> 
> For simple invocations, which do not supply a callback, a default callback
> is installed, which just sets nr_sets to 1 and transfers the number of
> spreadable vectors to the set_size array at index 0.
> 
> This is for now guarded by a check for nr_sets != 0 to keep the NVME driver
> working until it is converted to the callback mechanism.
> 
> To make sure that the driver configuration is correct under all circumstances
> the callback is invoked even when there are no interrupts for queues left,
> i.e. the pre/post requirements already exhaust the numner of available
> interrupts.
> 
> At the PCI layer irq_create_affinity_masks() has to be invoked even for the
> case where the legacy interrupt is used. That ensures that the callback is
> invoked and the device driver can adjust to that situation.
> 
> [ tglx: Fixed the simple case (no sets required). Moved the sanity check
>   	for nr_sets after the invocation of the callback so it catches
>   	broken drivers. Fixed the kernel doc comments for struct
>   	irq_affinity and de-'This patch'-ed the changelog ]
> 
> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

> @@ -1196,6 +1196,13 @@ int pci_alloc_irq_vectors_affinity(struc
>  	/* use legacy irq if allowed */
>  	if (flags & PCI_IRQ_LEGACY) {
>  		if (min_vecs == 1 && dev->irq) {
> +			/*
> +			 * Invoke the affinity spreading logic to ensure that
> +			 * the device driver can adjust queue configuration
> +			 * for the single interrupt case.
> +			 */
> +			if (affd)
> +				irq_create_affinity_masks(1, affd);

This looks like a leak because irq_create_affinity_masks() returns a
pointer to kcalloc()ed space, but we throw away the pointer.

Or is there something very subtle going on here, like this special
case doesn't allocate anything?  I do see the "Nothing to assign?"
case that returns NULL with no alloc, but it's not completely trivial
to verify that we take that case here.

>  			pci_intx(dev, 1);
>  			return 1;
>  		}

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