On 04/02/2025, John Stultz wrote: > On Wed, Apr 2, 2025 at 4:34 PM 'Will McVicker' via kernel-team > <kernel-team@xxxxxxxxxxx> wrote: > > > > To use the MCT as a sched_clock, the timer value has to be accessed vi > > an MCT register which is extremely slow. To improve performance on Arm64 > > SoCs, use the Arm architected timer as the default clocksource. Note, we > > Nit: sched_clock is sort of separate from the "default clocksource", > and after this patch we still register MCT as a clocksource, so this > doesn't sound quite right. > > I'd probably reword this slightly to: > "The MCT register is unfortunately very slow to access, but importantly > does not halt in the c2 idle state. So for ARM64, we can improve > performance by not registering the MCT for sched_clock, allowing the > system to use the faster ARM architected timer for sched_clock instead. > > The MCT is still registered as a clocksource, and a clockevent in order > to be a wakeup source for the arch_timer to exit the "c2" idle state. > > Since ARM32 SoCs don't have an architected timer, the MCT must continue > to be used for sched_clock. Detailed discussion on this topic can be > found at [1]. " Thanks John for the suggestion! I'll give more time for review feedback before sending the v3 with this update. Regards, Will > > > can't completely disable the MCT on Arm64 since it needs to be used as > > the wakeup source for the arch_timer to exit the "c2" idle state. > > > > Since ARM SoCs don't have an architectured timer, the MCT will continue > > to be the default clocksource. Detailed discussion on this topic can be > > found at [1]. > > > > [1] https://lore.kernel.org/linux-samsung-soc/1400188079-21832-1-git-send-email-chirantan@xxxxxxxxxxxx/ > > > > Signed-off-by: Donghoon Yu <hoony.yu@xxxxxxxxxxx> > > Signed-off-by: Youngmin Nam <youngmin.nam@xxxxxxxxxxx> > > [Original commit from https://android.googlesource.com/kernel/gs/+/630817f7080e92c5e0216095ff52f6eb8dd00727 > > Signed-off-by: Will McVicker <willmcvicker@xxxxxxxxxx> > > Reviewed-by: Youngmin Nam <youngmin.nam@xxxxxxxxxxx> > > Otherwise, looks good. > Acked-by: John Stultz <jstultz@xxxxxxxxxx> > > thanks > -john
