Re: i915.ko WC writes are slow after ea8596bb2d8d379

[Chris Wilson]

On Wed, Oct 08, 2014 at 05:10:59AM -0500, Chuck Ebbert wrote:
> On Wed, 8 Oct 2014 10:03:36 +0100
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > 
> > I ran into a problem on a Sandybridge i5-2500s whilst measuring the
> > performance of GTT write-combining access. I found subsequent runs were
> > about 10-40x slower than the first. For example,
> > 
> > igt/gem_gtt_speed:
> > 
> > Time to read 16k through a GTT map:             325.285µs
> > Time to write 16k through a GTT map:              4.729µs
> > Time to clear 16k through a GTT map:              4.584µs
> > Time to clear 16k through a cached GTT map:       1.342µs
> > 
> > on the second run became:
> > 
> > Time to read 16k through a GTT map:             332.148µs
> > Time to write 16k through a GTT map:            209.411µs
> > Time to clear 16k through a GTT map:             56.460µs
> > Time to clear 16k through a cached GTT map:      50.897µs
> > 
> > Naively I would say that we lost the wc on our ioremap.
> > /sys/kernel/debug/x86/pat_memtype_list remained the same across repeated
> > runs.
> > 
> > A bisection pointed to 
> > 
> > commit ea8596bb2d8d37957f3e92db9511c50801689180
> > Author: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> > Date:   Thu Jul 18 20:47:53 2013 +0900
> > 
> >     kprobes/x86: Remove unused text_poke_smp() and text_poke_smp_batch() functions
> > 
> > of which the active ingredient was just
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index b32ebf9..f4001e0 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -2334,7 +2334,6 @@ config HAVE_ATOMIC_IOMAP
> >  
> >  config HAVE_TEXT_POKE_SMP
> >         bool
> > -       select STOP_MACHINE if SMP
> >  
> >  config X86_DEV_DMA_OPS
> >         bool
> > 
> > and adding that back into the current build, e.g.
> 
> Hmm, set_mtrr() uses stop_machine(). I wonder if your MTRRs are out of
> sync and your results depend on which CPU the test runs on?

(From the other reply, it did and is still required).

I have run into other issues where stop_machine() tries to only do a
irq-disabled callback on the local CPU as opposed to halting all CPUs
and running the callback universally.

My understanding is that the root cause of the issue is:

diff --git a/init/Kconfig b/init/Kconfig
index af09b4f..8235e0b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1993,8 +1993,7 @@ config INIT_ALL_POSSIBLE
 
  config STOP_MACHINE
          bool
	  -       default y
	  -       depends on (SMP && MODULE_UNLOAD) || HOTPLUG_CPU
	  +       default y if SMP || HOTPLUG_CPU
	          help
		            Need stop_machine() primitive.

Although

diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
index d2abbdb..ff4f029 100644
--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -97,7 +97,7 @@ static inline int try_stop_cpus(const struct cpumask *cpumask,
  * grabbing every spinlock (and more).  So the "read" side to such a
  * lock is anything which disables preemption.
  */
-#if defined(CONFIG_STOP_MACHINE) && defined(CONFIG_SMP)
+#if defined(CONFIG_SMP) || defined(CONFIG_HOTPLUG_CPU)
 
 /**
  * stop_machine: freeze the machine on all CPUs and run this function
@@ -128,7 +128,7 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus);
 int stop_machine_from_inactive_cpu(int (*fn)(void *), void *data,
                                   const struct cpumask *cpus);
 
-#else   /* CONFIG_STOP_MACHINE && CONFIG_SMP */
+#else   /* CONFIG_SMP */
 
 static inline int __stop_machine(int (*fn)(void *), void *data,
                                 const struct cpumask *cpus)
@@ -153,5 +153,5 @@ static inline int stop_machine_from_inactive_cpu(int (*fn)(void *), void *data,
        return __stop_machine(fn, data, cpus);
 }
 
-#endif /* CONFIG_STOP_MACHINE && CONFIG_SMP */
+#endif /* CONFIG_SMP || CONFIG_HOTPLUG_CPU */
 #endif /* _LINUX_STOP_MACHINE */
diff --git a/init/Kconfig b/init/Kconfig
index af09b4f..44600a8 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1991,13 +1991,6 @@ config INIT_ALL_POSSIBLE
          it was better to provide this option than to break all the archs
          and have several arch maintainers pursuing me down dark alleys.
 
-config STOP_MACHINE
-       bool
-       default y
-       depends on (SMP && MODULE_UNLOAD) || HOTPLUG_CPU
-       help
-         Need stop_machine() primitive.
-
 source "block/Kconfig"
 
 config PREEMPT_NOTIFIERS
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index fd643d8..2dd1f306 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -513,7 +513,7 @@ static int __init cpu_stop_init(void)
 }
 early_initcall(cpu_stop_init);
 
-#ifdef CONFIG_STOP_MACHINE
+#if defined(CONFIG_SMP) || defined(CONFIG_HOTPLUG_CPU)
 
 int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 {
@@ -613,4 +613,4 @@ int stop_machine_from_inactive_cpu(int (*fn)(void *), void *data,
        return ret ?: done.ret;
 }
 
-#endif /* CONFIG_STOP_MACHINE */
+#endif /* CONFIG_SMP || CONFIG_HOTPLUG_CPU */

may be more apt.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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