Messages in this thread | | | Date | Fri, 27 Jul 2012 18:15:14 -0700 | Subject | Re: [RFC v2] ARM: sched_clock: update epoch_cyc on resume | From | Colin Cross <> |
| |
On Fri, Jul 27, 2012 at 4:38 PM, Russell King - ARM Linux <linux@arm.linux.org.uk> wrote: > On Sat, Jul 28, 2012 at 01:32:50AM +0200, Linus Walleij wrote: >> On Wed, Jul 25, 2012 at 4:49 AM, Colin Cross <ccross@android.com> wrote: >> >> > Many clocks that are used to provide sched_clock will reset during >> > suspend. If read_sched_clock returns 0 after suspend, sched_clock will >> > appear to jump forward. This patch resets cd.epoch_cyc to the current >> > value of read_sched_clock during resume, which causes sched_clock() just >> > after suspend to return the same value as sched_clock() just before >> > suspend. >> > >> > In addition, during the window where epoch_ns has been updated before >> > suspend, but epoch_cyc has not been updated after suspend, it is unknown >> > whether the clock has reset or not, and sched_clock() could return a >> > bogus value. Add a suspended flag, and return the pre-suspend epoch_ns >> > value during this period. >> > >> > The new behavior is triggered by calling setup_sched_clock_needs_suspend >> > instead of setup_sched_clock. >> > >> > Signed-off-by: Colin Cross <ccross@android.com> >> >> Sweet! >> Reviewed-by: Linus Walleij <linus.walleij@linaro.org> > > Have any of you looked at the patch I originally posted for doing this? > It needs updating but shows the overall principle - which is to ensure > that the epoch is up to date before suspending. > > It doesn't deal with resume, because different timers behave differently, > and there's no real way to deal with that properly. The important thing > that this patch does is to ensure sched_clock() doesn't ever go backwards. > > arch/arm/kernel/sched_clock.c | 18 ++++++++++++++++++ > 1 files changed, 18 insertions(+), 0 deletions(-) > > diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c > index 9a46370..4be4019 100644 > --- a/arch/arm/kernel/sched_clock.c > +++ b/arch/arm/kernel/sched_clock.c > @@ -10,6 +10,7 @@ > #include <linux/jiffies.h> > #include <linux/kernel.h> > #include <linux/sched.h> > +#include <linux/syscore_ops.h> > #include <linux/timer.h> > > #include <asm/sched_clock.h> > @@ -72,3 +73,20 @@ void __init sched_clock_postinit(void) > { > sched_clock_poll(sched_clock_timer.data); > } > + > +static int sched_clock_suspend(void) > +{ > + sched_clock_poll(sched_clock_timer.data); > + return 0; > +} > + > +static struct syscore_ops sched_clock_ops = { > + .suspend = sched_clock_suspend, > +}; > + > +static int __init sched_clock_syscore_init(void) > +{ > + register_syscore_ops(&sched_clock_ops); > + return 0; > +} > +device_initcall(sched_clock_syscore_init); >
That patch was merged in 3.4, and my patch is on top of it. Your patch updates epoch_cyc and epoch_ns in suspend, but if the first call to cyc_to_sched_clock after resume gets cyc = 0, cyc - epoch_cyc can be negative, although it will be cast back to a large positive number.
With my patch, epoch_cyc is updated in resume to whatever read_sched_clock() returns, and epoch_ns is still set to the suspend value, so it appears that sched_clock has not changed between sched_clock_suspend and sched_clock_resume. It will work with any timer behavior (reset to 0, reset to random value, or continuing counting). The old setup_sched_clock function maintains the old behavior to appease those who like their 32kHz sched clock to continue ticking in suspend, although I think it is more correct for all sched clocks to be faster than 32 kHz (when possible) and stop in suspend.
| |