linux-kernel - Re: [PATCH 1/3] sched/isolation: Add io

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Date: Tue, 25 Jun 2024 10:57:42 +0200
From: Daniel Wagner <dwagner@...e.de>
To: Ming Lei <ming.lei@...hat.com>
Cc: Hannes Reinecke <hare@...e.de>, Christoph Hellwig <hch@....de>, 
	Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>, 
	Sagi Grimberg <sagi@...mberg.me>, Frederic Weisbecker <fweisbecker@...e.com>, 
	Mel Gorman <mgorman@...e.de>, Sridhar Balaraman <sbalaraman@...allelwireless.com>, 
	"brookxu.cn" <brookxu.cn@...il.com>, linux-kernel@...r.kernel.org, linux-block@...r.kernel.org, 
	linux-nvme@...ts.infradead.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/3] sched/isolation: Add io_queue housekeeping option

On Tue, Jun 25, 2024 at 09:07:30AM GMT, Thomas Gleixner wrote:
> On Tue, Jun 25 2024 at 08:37, Hannes Reinecke wrote:
> > On 6/24/24 11:00, Daniel Wagner wrote:
> >> On Mon, Jun 24, 2024 at 10:47:05AM GMT, Christoph Hellwig wrote:
> >>>> Do you think we should introduce a new type or just use the existing
> >>>> managed_irq for this?
> >>>
> >>> No idea really.  What was the reason for adding a new one?
> >> 
> >> I've added the new type so that the current behavior of spreading the
> >> queues over to the isolated CPUs is still possible. I don't know if this
> >> a valid use case or not. I just didn't wanted to kill this feature it
> >> without having discussed it before.
> >> 
> >> But if we agree this doesn't really makes sense with isolcpus, then I
> >> think we should use the managed_irq one as nvme-pci is using the managed
> >> IRQ API.
> >> 
> > I'm in favour in expanding/modifying the managed irq case.
> > For managed irqs the driver will be running on the housekeeping CPUs 
> > only, and has no way of even installing irq handlers for the isolcpus.
> 
> Yes, that's preferred, but please double check with the people who
> introduced that in the first place.

The relevant code was added by Ming:

11ea68f553e2 ("genirq, sched/isolation: Isolate from handling managed
interrupts")

    [...] it can happen that a managed interrupt whose affinity
    mask contains both isolated and housekeeping CPUs is routed to an isolated
    CPU. As a consequence IO submitted on a housekeeping CPU causes interrupts
    on the isolated CPU.

    Add a new sub-parameter 'managed_irq' for 'isolcpus' and the corresponding
    logic in the interrupt affinity selection code.

    The subparameter indicates to the interrupt affinity selection logic that
    it should try to avoid the above scenario.
    [...]

>From the commit message I read the original indent is that managed_irq
should avoid speading queues on isolcated CPUs.

Ming, do you agree to use the managed_irq mask to limit the queue
spreading on isolated CPUs? It would make the io_queue option obsolete.