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Subject[PATCH] x86: enable preemption in delay

The RT team has been searching for a nasty latency. This latency shows
up out of the blue and has been seen to be as big as 5ms!

Using ftrace I found the cause of the latency.

pcscd-2995 3dNh1 52360300us : irq_exit (smp_apic_timer_interrupt)
pcscd-2995 3dN.2 52360301us : idle_cpu (irq_exit)
pcscd-2995 3dN.2 52360301us : rcu_irq_exit (irq_exit)
pcscd-2995 3dN.1 52360771us : smp_apic_timer_interrupt (apic_timer_interrupt
)
pcscd-2995 3dN.1 52360771us : exit_idle (smp_apic_timer_interrupt)

Here's an example of a 400 us latency. pcscd took a timer interrupt and
returned with "need resched" enabled, but did not reschedule until after
the next interrupt came in at 52360771us 400us later!

At first I thought we somehow missed a preemption check in entry.S. But
I also noticed that this always seemed to happen during a __delay call.

pcscd-2995 3dN.2 52360836us : rcu_irq_exit (irq_exit)
pcscd-2995 3.N.. 52361265us : preempt_schedule (__delay)

Looking at the x86 delay, I found my problem.

In git commit 35d5d08a085c56f153458c3f5d8ce24123617faf, Andrew Morton
placed preempt_disable around the entire delay due to TSC's not working
nicely on SMP. Unfortunately for those that care about latencies this
is devastating! Especially when we have callers to mdelay(8).

Here I enable preemption during the loop and account for anytime the task
migrates to a new CPU. The delay asked for may be extended a bit by
the migration, but delay only guarantees that it will delay for that minimum
time. Delaying longer should not be an issue.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
---
arch/x86/lib/delay_32.c | 23 ++++++++++++++++++++++-
arch/x86/lib/delay_64.c | 26 +++++++++++++++++++++++---
2 files changed, 45 insertions(+), 4 deletions(-)
Index: linux-compile.git/arch/x86/lib/delay_32.c
===================================================================
--- linux-compile.git.orig/arch/x86/lib/delay_32.c 2008-05-24 22:30:49.000000000 -0400
+++ linux-compile.git/arch/x86/lib/delay_32.c 2008-05-24 22:31:43.000000000 -0400
@@ -44,12 +44,33 @@ static void delay_loop(unsigned long loo
static void delay_tsc(unsigned long loops)
{
unsigned long bclock, now;
+ int cpu;

- preempt_disable(); /* TSC's are per-cpu */
+ preempt_disable();
+ cpu = smp_processor_id();
rdtscl(bclock);
do {
rep_nop();
rdtscl(now);
+ /* Allow RT tasks to run */
+ preempt_enable();
+ preempt_disable();
+ /*
+ * It is possible that we moved to another CPU,
+ * and since TSC's are per-cpu we need to
+ * calculate that. The delay must guarantee that
+ * we wait "at least" the amount of time. Being
+ * moved to another CPU could make the wait longer
+ * but we just need to make sure we waited long
+ * enough. Rebalance the counter for this CPU.
+ */
+ if (unlikely(cpu != smp_processor_id())) {
+ if ((now-bclock) >= loops)
+ break;
+ loops -= (now - bclock);
+ rdtscl(bclock);
+ rdtscl(now);
+ }
} while ((now-bclock) < loops);
preempt_enable();
}
Index: linux-compile.git/arch/x86/lib/delay_64.c
===================================================================
--- linux-compile.git.orig/arch/x86/lib/delay_64.c 2008-05-24 22:30:49.000000000 -0400
+++ linux-compile.git/arch/x86/lib/delay_64.c 2008-05-24 22:31:43.000000000 -0400
@@ -31,14 +31,34 @@ int __devinit read_current_timer(unsigne
void __delay(unsigned long loops)
{
unsigned bclock, now;
+ int cpu;

- preempt_disable(); /* TSC's are pre-cpu */
+ preempt_disable();
+ cpu = smp_processor_id();
rdtscl(bclock);
do {
rep_nop();
rdtscl(now);
- }
- while ((now-bclock) < loops);
+ /* Allow RT tasks to run */
+ preempt_enable();
+ preempt_disable();
+ /*
+ * It is possible that we moved to another CPU,
+ * and since TSC's are per-cpu we need to
+ * calculate that. The delay must guarantee that
+ * we wait "at least" the amount of time. Being
+ * moved to another CPU could make the wait longer
+ * but we just need to make sure we waited long
+ * enough. Rebalance the counter for this CPU.
+ */
+ if (unlikely(cpu != smp_processor_id())) {
+ if ((now-bclock) >= loops)
+ break;
+ loops -= (now - bclock);
+ rdtscl(bclock);
+ rdtscl(now);
+ }
+ } while ((now-bclock) < loops);
preempt_enable();
}
EXPORT_SYMBOL(__delay);


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