linux-kernel - Re: [patch v6 7/7] genirq/affinity: Add support for non-managed affinity sets

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Message-ID: <alpine.DEB.2.21.1902172010060.1683@nanos.tec.linutronix.de>
Date:   Sun, 17 Feb 2019 20:17:05 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ming Lei <ming.lei@...hat.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Bjorn Helgaas <helgaas@...nel.org>,
        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 7/7] genirq/affinity: Add support for non-managed
 affinity sets

On Sun, 17 Feb 2019, Ming Lei wrote:
> On Sat, Feb 16, 2019 at 06:13:13PM +0100, Thomas Gleixner wrote:
> > Some drivers need an extra set of interrupts which should not be marked
> > managed, but should get initial interrupt spreading.
> 
> Could you share the drivers and their use case?

You were Cc'ed on that old discussion:

 https://lkml.kernel.org/r/300d6fef733ca76ced581f8c6304bac6@mail.gmail.com

> > For both interrupt sets the interrupts are properly spread out, but the
> > second set is not marked managed.
> 
> Given drivers only care the managed vs non-managed interrupt numbers,
> just wondering why this case can't be covered by .pre_vectors &
> .post_vectors?

Well, yes, but post/pre are not subject to spreading and I really don't
want to go there.

> Also this kind of usage may break blk-mq easily, in which the following
> rule needs to be respected:
> 
> 1) all CPUs are required to spread among each interrupt set
> 
> 2) no any CPU is shared between two IRQs in same set.

I don't see how that would break blk-mq. The unmanaged set is not used by
the blk-mq stuff, that's some driver internal voodoo. So blk-mq still gets
a perfectly spread and managed interrupt set for the queues.

> >  	for (i = 0, usedvecs = 0; i < affd->nr_sets; i++) {
> > -		unsigned int this_vecs = affd->set_size[i];
> > +		bool managed = affd->unmanaged_sets & (1U << i) ? true : false;
> 
> The above check is inverted.

Doh. Stupid me.

Thanks,

	tglx