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SubjectRe: [PATCH 13/15] sched: Use a static_key for sched_clock_stable
On 12/12/2013 09:08 AM, Peter Zijlstra wrote:
> In order to avoid the runtime condition and variable load turn
> sched_clock_stable into a static_key.
>
> Also provide a shorter implementation of local_clock() and
> cpu_clock(int) when sched_clock_stable==1.
>
> MAINLINE PRE POST
>
> sched_clock_stable: 1 1 1
> (cold) sched_clock: 329841 221876 215295
> (cold) local_clock: 301773 234692 220773
> (warm) sched_clock: 38375 25602 25659
> (warm) local_clock: 100371 33265 27242
> (warm) rdtsc: 27340 24214 24208
> sched_clock_stable: 0 0 0
> (cold) sched_clock: 382634 235941 237019
> (cold) local_clock: 396890 297017 294819
> (warm) sched_clock: 38194 25233 25609
> (warm) local_clock: 143452 71234 71232
> (warm) rdtsc: 27345 24245 24243
>
> Signed-off-by: Peter Zijlstra<peterz@infradead.org>

Hi Peter,

This patch seems to be causing an issue with booting a KVM guest. It seems that it
causes the time to go random during early boot process:

[ 0.000000] Initmem setup node 30 [mem 0x12ee000000-0x138dffffff]
[ 0.000000] NODE_DATA [mem 0xcfa42000-0xcfa72fff]
[ 0.000000] NODE_DATA(30) on node 1
[ 0.000000] Initmem setup node 31 [mem 0x138e000000-0x142fffffff]
[ 0.000000] NODE_DATA [mem 0xcfa11000-0xcfa41fff]
[ 0.000000] NODE_DATA(31) on node 1
[ 0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
[ 0.000000] kvm-clock: cpu 0, msr 0:cf991001, boot clock
[133538.294040] Zone ranges:
[133538.294338] DMA [mem 0x00001000-0x00ffffff]
[133538.294804] DMA32 [mem 0x01000000-0xffffffff]
[133538.295223] Normal [mem 0x100000000-0x142fffffff]
[133538.295670] Movable zone start for each node

Looking at the code, initially I though that the problem is with:

+void set_sched_clock_stable(void)
+{
+ if (!sched_clock_stable())
+ static_key_slow_dec(&__sched_clock_stable);
+}
+
+void clear_sched_clock_stable(void)
+{
+ /* XXX worry about clock continuity */
+ if (sched_clock_stable())
+ static_key_slow_inc(&__sched_clock_stable);
+}

I think the jump label inc/dec is reversed here. We would want to inc it when enabling
and dec when disabling, no?
However, trying to reverse the two didn't help. I was still seeing the same odd behaviour.

I tried doing a simple conversion to using a simple var like before, which looks like this:

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 6bd6a67..a035932 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -76,26 +76,21 @@ EXPORT_SYMBOL_GPL(sched_clock);
__read_mostly int sched_clock_running;

#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
-static struct static_key __sched_clock_stable = STATIC_KEY_INIT;
+static int __sched_clock_stable;

int sched_clock_stable(void)
{
- if (static_key_false(&__sched_clock_stable))
- return false;
- return true;
+ return __sched_clock_stable;
}

void set_sched_clock_stable(void)
{
- if (!sched_clock_stable())
- static_key_slow_dec(&__sched_clock_stable);
+ __sched_clock_stable = 1;
}

static void __clear_sched_clock_stable(struct work_struct *work)
{
- /* XXX worry about clock continuity */
- if (sched_clock_stable())
- static_key_slow_inc(&__sched_clock_stable);
+ __sched_clock_stable = 0;
}

static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);
@@ -340,7 +335,7 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
*/
u64 cpu_clock(int cpu)
{
- if (static_key_false(&__sched_clock_stable))
+ if (!sched_clock_stable())
return sched_clock_cpu(cpu);

return sched_clock();
@@ -355,7 +350,7 @@ u64 cpu_clock(int cpu)
*/
u64 local_clock(void)
{
- if (static_key_false(&__sched_clock_stable))
+ if (!sched_clock_stable())
return sched_clock_cpu(raw_smp_processor_id());

return sched_clock();

This has corrected the issue:

[ 0.000000] Initmem setup node 31 [mem 0x138e000000-0x142fffffff]
[ 0.000000] NODE_DATA [mem 0xcfa11000-0xcfa41fff]
[ 0.000000] NODE_DATA(31) on node 1
[ 0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
[ 0.000000] kvm-clock: cpu 0, msr 0:cf991001, boot clock
[ 0.000000] Zone ranges:
[ 0.000000] DMA [mem 0x00001000-0x00ffffff]
[ 0.000000] DMA32 [mem 0x01000000-0xffffffff]
[ 0.000000] Normal [mem 0x100000000-0x142fffffff]
[ 0.000000] Movable zone start for each node
[ 0.000000] Early memory node ranges
[ timing is correct for the rest of the boot]
At this point, I thought that there's something up with jump labels being used this early (?) and
tried compiling with CONFIG_JUMP_LABELS=n, this didn't solve the issue.

This makes me thing there's something different related to jumplabels we're missing, as the
no-jumplabel config should be very similar to the patch I did above, I just can't figure
out what it is.


Thanks,
Sasha


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