linux-kernel - Re: [PATCH v2 -tip 1/5] x86, MSI: Support multiple MSIs in presense of IRQ remapping

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Message-ID: <CAE9FiQUuAPy884Ytu5PrFkcoQ5tT_DPxyYDk1D1jKbo+G577Mw@mail.gmail.com>
Date:	Mon, 3 Sep 2012 11:53:39 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Jeff Garzik <jgarzik@...ox.com>,
	Matthew Wilcox <willy@...ux.intel.com>, x86@...nel.org,
	linux-pci@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: [PATCH v2 -tip 1/5] x86, MSI: Support multiple MSIs in presense
 of IRQ remapping

On Mon, Sep 3, 2012 at 2:17 AM, Alexander Gordeev <agordeev@...hat.com> wrote:
> The MSI specification has several constraints in comparison with MSI-X,
> most notable of them is the inability to configure MSIs independently.
> As a result, it is impossible to dispatch interrupts from different
> queues to different CPUs. This is largely devalues the support of
> multiple MSIs in SMP systems.
>
> Also, a necessity to allocate a contiguous block of vector numbers for
> devices capable of multiple MSIs might cause a considerable pressure on
> x86 interrupt vector allocator and could lead to fragmentation of the
> interrupt vectors space.
>
> This patch overcomes both drawbacks in presense of IRQ remapping and
> lets devices take advantage of multiple queues and per-IRQ affinity
> assignments.
>
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> ---
>  arch/x86/kernel/apic/io_apic.c |  166 +++++++++++++++++++++++++++++++++++++--
>  include/linux/irq.h            |    6 ++
>  kernel/irq/chip.c              |   30 +++++--
>  kernel/irq/irqdesc.c           |   31 ++++++++
>  4 files changed, 216 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
> index c265593..5fd2577 100644
> --- a/arch/x86/kernel/apic/io_apic.c
> +++ b/arch/x86/kernel/apic/io_apic.c
> @@ -305,6 +305,11 @@ static int alloc_irq_from(unsigned int from, int node)
>         return irq_alloc_desc_from(from, node);
>  }
>
> +static int alloc_irqs_from(unsigned int from, unsigned int count, int node)
> +{
> +       return irq_alloc_descs_from(from, count, node);
> +}
> +
>  static void free_irq_at(unsigned int at, struct irq_cfg *cfg)
>  {
>         free_irq_cfg(at, cfg);
> @@ -3039,6 +3044,55 @@ int create_irq(void)
>         return irq;
>  }
>
> +unsigned int create_irqs(unsigned int from, unsigned int count, int node)
> +{
> +       struct irq_cfg **cfg;
> +       unsigned long flags;
> +       int irq, i;
> +
> +       if (from < nr_irqs_gsi)
> +               from = nr_irqs_gsi;
> +
> +       cfg = kzalloc_node(count * sizeof(cfg[0]), GFP_KERNEL, node);
> +       if (!cfg)
> +               return 0;
> +
> +       irq = alloc_irqs_from(from, count, node);
> +       if (irq < 0)
> +               goto out_cfgs;
> +
> +       for (i = 0; i < count; i++) {
> +               cfg[i] = alloc_irq_cfg(irq + i, node);
> +               if (!cfg[i])
> +                       goto out_irqs;
> +       }
> +
> +       raw_spin_lock_irqsave(&vector_lock, flags);
> +       for (i = 0; i < count; i++)
> +               if (__assign_irq_vector(irq + i, cfg[i], apic->target_cpus()))
> +                       goto out_vecs;
> +       raw_spin_unlock_irqrestore(&vector_lock, flags);
> +
> +       for (i = 0; i < count; i++) {
> +               irq_set_chip_data(irq + i, cfg[i]);
> +               irq_clear_status_flags(irq + i, IRQ_NOREQUEST);
> +       }
> +
> +       kfree(cfg);
> +       return irq;
> +
> +out_vecs:
> +       for (; i; i--)
> +               __clear_irq_vector(irq + i - 1, cfg[i - 1]);
> +       raw_spin_unlock_irqrestore(&vector_lock, flags);
> +out_irqs:
> +       for (i = 0; i < count; i++)
> +               free_irq_at(irq + i, cfg[i]);
> +out_cfgs:
> +       kfree(cfg);
> +       return 0;
> +}
> +

You may update create_irq_nr to be __create_irq_nr, and it could take
extra count.

and later have create_irq_nr to be __create_irq_nr(,1,)
and create_irqs to be __create_irq_nr(,count,)
....

BTW, in short, how much performance benefits for adding 500 lines code?

Thanks

Yinghai
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