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SubjectRe: CONFIG_NO_HZ breaks blktrace timestamps

* Guillaume Chazarain <guichaz@yahoo.fr> wrote:

> David Dillow <dillowda@ornl.gov> wrote:
>
> > Patched kernel, nohz=off:
> > .clock_underflows : 213887
>
> A little bit of warning about these patches, they are WIP, that's why
> I did not send them earlier. It regress nohz=off.

ok. I have applied all but this one:

> A bit of context: these patches aim at making sure cpu_clock() on my
> laptop (cpufreq enabled) never overflows/underflows/warps with
> CONFIG_NOHZ enabled. With these patches, I have a few hundreds
> overflows and underflows during early bootup, and then nothing :-)

cool :-)

> > sched: Fix rq->clock overflows detection with CONFIG_NO_HZ
>
> I think this one is the most important for David, but unfortunately it
> has some problems.
>
> > +static inline u64 max_skipped_ticks(struct rq *rq)
> > +{
> > + return nohz_on(cpu_of(rq)) ? jiffies - rq->last_tick_seen + 2 : 1;
> > +}
>
> Here, I initially wrote rq->last_tick_seen + 1 but experiments showed
> that +2 was needed as I really saw deltas of 2 milliseconds.
>
> These patches have two objectives:
> - taking into account that jiffies are not always incremented by 1
> thanks to nohz
> - as the tick is stopped and restarted it may not tick at the exact
> expected moment, so allow a window of 1 jiffie. If the tick occurs
> during the right jiffy, we know the TSC is more precise than the tick
> so don't correct the clock.

i think it's much simpler to do what i have below. Could you try it on
your box? Or if it is using ACPI idle - in that case the callbacks
should already be there and there should be no need for further fixups.

> And the problem is that I seem to need a window of 2 jiffies, so I need
> some help.
>
> > sched: make sure jiffies is up to date before calling __update_rq_clock()
>
> This is one is needed too but I'm less confident in its validity.
>
> > scheduler_tick() is not called every jiffies
>
> This one is a bit ugly and seems to break nohz=off.

ok, i took this one out.

Ingo

-------------------->
Subject: x86: idle wakeup event in the HLT loop
From: Ingo Molnar <mingo@elte.hu>
do a proper idle-wakeup event on HLT as well - some CPUs stop the TSC
in HLT too, not just when going through the ACPI methods.

(the ACPI idle code already does this.)

[ update the 64-bit side too, as noticed by Jiri Slaby. ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
arch/x86/kernel/process_32.c | 15 ++++++++++++---
arch/x86/kernel/process_64.c | 13 ++++++++++---
2 files changed, 22 insertions(+), 6 deletions(-)
Index: linux-x86.q/arch/x86/kernel/process_32.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/process_32.c
+++ linux-x86.q/arch/x86/kernel/process_32.c
@@ -113,10 +113,19 @@ void default_idle(void)
smp_mb();

local_irq_disable();
- if (!need_resched())
+ if (!need_resched()) {
+ ktime_t t0, t1;
+ u64 t0n, t1n;
+
+ t0 = ktime_get();
+ t0n = ktime_to_ns(t0);
safe_halt(); /* enables interrupts racelessly */
- else
- local_irq_enable();
+ local_irq_disable();
+ t1 = ktime_get();
+ t1n = ktime_to_ns(t1);
+ sched_clock_idle_wakeup_event(t1n - t0n);
+ }
+ local_irq_enable();
current_thread_info()->status |= TS_POLLING;
} else {
/* loop is done by the caller */
Index: linux-x86.q/arch/x86/kernel/process_64.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/process_64.c
+++ linux-x86.q/arch/x86/kernel/process_64.c
@@ -116,9 +116,16 @@ static void default_idle(void)
smp_mb();
local_irq_disable();
if (!need_resched()) {
- /* Enables interrupts one instruction before HLT.
- x86 special cases this so there is no race. */
- safe_halt();
+ ktime_t t0, t1;
+ u64 t0n, t1n;
+
+ t0 = ktime_get();
+ t0n = ktime_to_ns(t0);
+ safe_halt(); /* enables interrupts racelessly */
+ local_irq_disable();
+ t1 = ktime_get();
+ t1n = ktime_to_ns(t1);
+ sched_clock_idle_wakeup_event(t1n - t0n);
} else
local_irq_enable();
current_thread_info()->status |= TS_POLLING;

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