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    SubjectRe: [PATCH v3 03/16] arm64: Allow IPIs to be handled as normal interrupts
    On 2020-10-27 10:12, Vincent Guittot wrote:
    > HI Marc,
    >
    > On Mon, 19 Oct 2020 at 17:43, Vincent Guittot
    > <vincent.guittot@linaro.org> wrote:
    >>
    >> On Mon, 19 Oct 2020 at 15:04, Marc Zyngier <maz@kernel.org> wrote:
    >> >
    >
    > ...
    >
    >> > >>
    >> > >> One of the major difference is that we end up, in some cases
    >> > >> (such as when performing IRQ time accounting on the scheduler
    >> > >> IPI), end up with nested irq_enter()/irq_exit() pairs.
    >> > >> Other than the (relatively small) overhead, there should be
    >> > >> no consequences to it (these pairs are designed to nest
    >> > >> correctly, and the accounting shouldn't be off).
    >> > >
    >> > > While rebasing on mainline, I have faced a performance regression for
    >> > > the benchmark:
    >> > > perf bench sched pipe
    >> > > on my arm64 dual quad core (hikey) and my 2 nodes x 112 CPUS (thx2)
    >> > >
    >> > > The regression comes from:
    >> > > commit: d3afc7f12987 ("arm64: Allow IPIs to be handled as normal
    >> > > interrupts")
    >> >
    >> > That's interesting, as this patch doesn't really change anything (most
    >> > of the potential overhead comes in later). The only potential overhead
    >> > I can see is that the scheduler_ipi() call is now wrapped around
    >> > irq_enter()/irq_exit().
    >> >
    >> > >
    >> > > v5.9 + this patch
    >> > > hikey : 48818(+/- 0.31) 37503(+/- 0.15%) -23.2%
    >> > > thx2 : 132410(+/- 1.72) 122646(+/- 1.92%) -7.4%
    >> > >
    >> > > By + this patch, I mean merging branch from this patch. Whereas
    >> > > merging the previous:
    >> > > commit: 83cfac95c018 ("genirq: Allow interrupts to be excluded from
    >> > > /proc/interrupts")
    >> > > It doesn't show any regression
    >> >
    >> > Since you are running perf, can you spot where the overhead occurs?
    >
    > Any idea about the root cause of the regression ?
    > I have faced it on more arm64 platforms in the meantime

    two possible causes:

    (1) irq_enter/exit on the rescheduling IPI means we reschedule much more
    often
    (2) irq_domain lookups add some overhead.

    For (1), I have this series[1] which is ugly as sin and needs much more
    testing.

    For (2), I have some ideas which need more work (let the irq domain
    resolve to
    an irq_desc instead of an interrupt number, avoiding another radix-tree
    lookup).

    M.

    [1]
    https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=irq/ipi-fixes
    --
    Jazz is not dead. It just smells funny...

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