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Date: Wed, 22 Jan 2014 18:14:54 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Sasha Levin <sasha.levin@...cle.com>
Cc: Arjan van de Ven <arjan@...ux.intel.com>, lenb@...nel.org,
rjw@...ysocki.net, Eliezer Tamir <eliezer.tamir@...ux.intel.com>,
rui.zhang@...el.com, jacob.jun.pan@...ux.intel.com,
Mike Galbraith <bitbucket@...ine.de>,
Ingo Molnar <mingo@...nel.org>, hpa@...or.com,
paulmck@...ux.vnet.ibm.com, Thomas Gleixner <tglx@...utronix.de>,
John Stultz <john.stultz@...aro.org>,
Andy Lutomirski <luto@...capital.net>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/15] sched: Use a static_key for sched_clock_stable
On Tue, Jan 21, 2014 at 05:28:37PM -0500, Sasha Levin wrote:
> [ 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
OK, took me a while to fiddle with KVM and all the various muck around
that to reproduce. But I can confirm the below does fix the issue for
me.
I'm hoping to not have to re-introcude the kevents_up() check, but I
need to figure out what hardware triggered that and test again.
---
Subject: sched/clock: Fixup early sched_clock initialization
From: Peter Zijlstra <peterz@...radead.org>
Date: Wed, 22 Jan 2014 12:59:18 +0100
The code would assume sched_clock_stable() and switch to !stable
later, this switch brings a discontinuity in time.
The discontinuity on switching from stable to unstable was always
present, but previously we would set stable/unstable before
initializing TSC and usually stick to the one we start out with.
So the static_key bits brought an extra switch where there previously
wasn't one.
Things are further complicated by the fact that we cannot use
static_key as early as we usually call set_sched_clock_stable().
Fix things by tracking the stable state in a regular variable and only
set the static_key to the right state on sched_clock_init(), which is
ran right after late_time_init->tsc_init().
Before this we would not be using the TSC anyway.
Fixes: 35af99e646c7 ("sched/clock, x86: Use a static_key for sched_clock_stable")
Cc: jacob.jun.pan@...ux.intel.com
Cc: Mike Galbraith <bitbucket@...ine.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: hpa@...or.com
Cc: paulmck@...ux.vnet.ibm.com
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: John Stultz <john.stultz@...aro.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: lenb@...nel.org
Cc: rjw@...ysocki.net
Cc: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Cc: rui.zhang@...el.com
Reported-by: Sasha Levin <sasha.levin@...cle.com>
Reported-by: dyoung@...hat.com
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/20140122115918.GG3694@twins.programming.kicks-ass.net
---
kernel/sched/clock.c | 53 +++++++++++++++++++++++++++++++++++++++------------
1 file changed, 41 insertions(+), 12 deletions(-)
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -77,35 +77,50 @@ __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_early;
int sched_clock_stable(void)
{
- if (static_key_false(&__sched_clock_stable))
- return false;
- return true;
+ return static_key_false(&__sched_clock_stable);
}
-void set_sched_clock_stable(void)
+static void __set_sched_clock_stable(void)
{
if (!sched_clock_stable())
- static_key_slow_dec(&__sched_clock_stable);
+ static_key_slow_inc(&__sched_clock_stable);
+}
+
+void set_sched_clock_stable(void)
+{
+ __sched_clock_stable_early = 1;
+
+ smp_mb(); /* matches sched_clock_init() */
+
+ if (!sched_clock_running)
+ return;
+
+ __set_sched_clock_stable();
}
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);
+ static_key_slow_dec(&__sched_clock_stable);
}
static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);
void clear_sched_clock_stable(void)
{
- if (keventd_up())
- schedule_work(&sched_clock_work);
- else
- __clear_sched_clock_stable(&sched_clock_work);
+ __sched_clock_stable_early = 0;
+
+ smp_mb(); /* matches sched_clock_init() */
+
+ if (!sched_clock_running)
+ return;
+
+ schedule_work(&sched_clock_work);
}
struct sched_clock_data {
@@ -140,6 +155,20 @@ void sched_clock_init(void)
}
sched_clock_running = 1;
+
+ /*
+ * Ensure that it is impossible to not do a static_key update.
+ *
+ * Either {set,clear}_sched_clock_stable() must see sched_clock_running
+ * and do the update, or we must see their __sched_clock_stable_early
+ * and do the update, or both.
+ */
+ smp_mb(); /* matches {set,clear}_sched_clock_stable() */
+
+ if (__sched_clock_stable_early)
+ __set_sched_clock_stable();
+ else
+ __clear_sched_clock_stable(NULL);
}
/*
@@ -340,7 +369,7 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_wakeu
*/
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 +384,7 @@ 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();
--
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