On Tue, 27 Oct 2020 at 11:37, Marc Zyngier <m...@kernel.org> wrote: > > On 2020-10-27 10:12, Vincent Guittot wrote: > > HI Marc, > > > > On Mon, 19 Oct 2020 at 17:43, Vincent Guittot > > <vincent.guit...@linaro.org> wrote: > >> > >> On Mon, 19 Oct 2020 at 15:04, Marc Zyngier <m...@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.
Ok, I'm going to test this series to see if it fixes the perf regression > > 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...