Messages in this thread | | | Date | Tue, 27 Oct 2020 10:37:35 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH v3 03/16] arm64: Allow IPIs to be handled as normal interrupts |
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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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