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    SubjectRe: [PATCH] x86: Add check for number of available vectors before CPU down [v2]
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    On 12/29/13, Prarit Bhargava <prarit@redhat.com> wrote:
    >
    >
    > On 12/20/2013 04:41 AM, rui wang wrote:
    <<snip>>
    >> The vector number for an irq is programmed in the LSB of the IOAPIC
    >> IRTE (or MSI data register in the case of MSI/MSIx). So there can be
    >> only one vector number (although multiple CPUs can be specified
    >> through DM). An MSI-capable device can dynamically change the lower
    >> few bits in the LSB to signal multiple interrupts with a contiguous
    >> range of vectors in powers of 2,but each of these vectors is treated
    >> as a separate IRQ. i.e. each of them has a separate irq desc, or a
    >> separate line in the /proc/interrupt file. This patch shows the MSI
    >> irq allocation in detail:
    >> http://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?id=51906e779f2b13b38f8153774c4c7163d412ffd9
    >>
    >> Thanks
    >> Rui
    >>
    >
    > Gong and Rui,
    >
    > After looking at this in detail I realized I made a mistake in my patch by
    > including the check for the smp_affinity. Simply put, it shouldn't be
    > there
    > given Rui's explanation above.
    >
    > So I think the patch simply needs to do:
    >
    > this_count = 0;
    > for (vector = FIRST_EXTERNAL_VECTOR; vector < NR_VECTORS; vector++)
    > {
    > irq = __this_cpu_read(vector_irq[vector]);
    > if (irq >= 0) {
    > desc = irq_to_desc(irq);
    > data = irq_desc_get_irq_data(desc);
    > affinity = data->affinity;
    > if (irq_has_action(irq) && !irqd_is_per_cpu(data))
    > this_count++;
    > }
    > }
    >
    > Can the two of you confirm the above is correct? It would be greatly
    > appreciated.

    An irq can be mapped to only one vector number, but can have multiple
    destination CPUs. i.e. the same irq/vector can appear on multiple
    CPUs' vector_irq[]. So checking data->affinity is necessary I think.
    But notice that data->affinity is updated in chip->irq_set_affinity()
    inside fixup_irqs(), while cpu_online_mask is updated in
    remove_cpu_from_maps() inside cpu_disable_common(). They are updated
    in different places. So the algorithm to check them against each other
    should be different, depending on where you put the check_vectors().
    That's my understanding.

    Thanks
    Rui


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