Messages in this thread | | | Date | Fri, 27 Jul 2012 20:30:31 -0700 | Subject | Re: [RFC v2] ARM: sched_clock: update epoch_cyc on resume | From | Colin Cross <> |
| |
On Fri, Jul 27, 2012 at 6:15 PM, Colin Cross <ccross@android.com> wrote: > 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.
I think the existing code can cause sched_clock to go backwards if the register read by the read function resets to 0 in suspend:
In sched_clock_suspend epoch_cyc is updated to 0x00002000.
The first sched_clock call after resume gets cyc = 0, returning epoch_ns + cyc_to_ns(0xffffe000)
Later, sched_clock gets cyc = 0x5000, returning epoch_ns + cyc_to_ns(0x3000), which will be much smaller than the previous sched_clock value.
| |