| Message ID | 20120727233829.GB14835@n2100.arm.linux.org.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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.
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.
On Fri, Jul 27, 2012 at 8:30 PM, Colin Cross <ccross@android.com> wrote: > On Fri, Jul 27, 2012 at 6:15 PM, Colin Cross <ccross@android.com> wrote: >> 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. Russell, any update on this? Should sched_clock.c handle read functions that go backwards in suspend, or should I modify the read function to track an offset in suspend and always return a monotonically increasing value across suspend?
On Fri, Aug 3, 2012 at 3:28 AM, Colin Cross <ccross@android.com> wrote: > On Fri, Jul 27, 2012 at 8:30 PM, Colin Cross <ccross@android.com> wrote: >> On Fri, Jul 27, 2012 at 6:15 PM, Colin Cross <ccross@android.com> wrote: >>> 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. > > Russell, any update on this? Should sched_clock.c handle read > functions that go backwards in suspend, or should I modify the read > function to track an offset in suspend and always return a > monotonically increasing value across suspend? Colin, I suggest you put this into Russell's patch tracker so he can keep track of it from there. Yours, Linus Walleij
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);