Re: [RFC][PATCH 0/5] preempt_count rework

Peter Zijlstra
2013-08-14T16:04:31Z

On Wed, Aug 14, 2013 at 06:47:45AM -0700, H. Peter Anvin wrote:

> I still don't see this using a decrement of the percpu variable
> anywhere. The C compiler doesn't know how to generate those, so if I'm
> not completely wet we will end up relying on sub_preempt_count()...
> which, because it relies on taking the address of the percpu variable
> will generate absolutely horrific code.
>
> On x86, you never want to take the address of a percpu variable if you
> can avoid it, as you end up generating code like:
>
> movq %fs:0,%rax
> subl $1,(%rax)
>

Urgh,. yes you're right. I keep forgetting GCC doesn't know how to merge
those :/

OK, so something like the below would cure the worst of that I suppose.
It compiles but doesn't boot; must've done something wrong.

Someone please look at it because my asm-foo blows. I pretty much
copy/pasted this from asm/percpu.h.

---
--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -20,6 +20,28 @@ static __always_inline int *preempt_coun
return &__raw_get_cpu_var(__preempt_count);
}

+#define __preempt_count_add(x) do { \
+ asm("addl %1," __percpu_arg(0) \
+ : "+m" (__preempt_count) \
+ : "ri" ((int)x) \
+ : "memory"); \
+} while (0)
+
+#define __preempt_count_sub(x) do { \
+ asm("subl %1," __percpu_arg(0) \
+ : "+m" (__preempt_count) \
+ : "ri" ((int)x) \
+ : "memory"); \
+} while (0)
+
+#define preempt_enable() do { \
+ asm("\nsubl $1," __percpu_arg(0) \
+ "\njnz 1f" \
+ "\ncall preempt_schedule" \
+ "\n1:" : "+m" (__preempt_count) \
+ : : "memory"); \
+} while (0)
+
/*
* must be macros to avoid header recursion hell
*/
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -17,6 +17,9 @@ static __always_inline int *preempt_coun
return &current_thread_info()->preempt_count;
}

+#define __preempt_count_add(x) do { current_thread_info()->preempt_count += (x); } while (0)
+#define __preempt_count_sub(x) __preempt_count_add(-(x))
+
/*
* must be macros to avoid header recursion hell
*/
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -45,8 +45,8 @@ static __always_inline bool test_preempt
extern void add_preempt_count(int val);
extern void sub_preempt_count(int val);
#else
-# define add_preempt_count(val) do { *preempt_count_ptr() += (val); } while (0)
-# define sub_preempt_count(val) do { *preempt_count_ptr() -= (val); } while (0)
+# define add_preempt_count(val) __preempt_count_add(val)
+# define sub_preempt_count(val) __preempt_count_sub(val)
#endif

#define inc_preempt_count() add_preempt_count(1)
@@ -101,17 +101,17 @@ do { \

#define preempt_enable_no_resched() sched_preempt_enable_no_resched()

+#ifndef preempt_enable
#define preempt_enable() \
do { \
preempt_enable_no_resched(); \
preempt_check_resched(); \
} while (0)
+#endif

/* For debugging and tracer internals only! */
-#define add_preempt_count_notrace(val) \
- do { *preempt_count_ptr() += (val); } while (0)
-#define sub_preempt_count_notrace(val) \
- do { *preempt_count_ptr() -= (val); } while (0)
+#define add_preempt_count_notrace(val) __preempt_count_add(val)
+#define sub_preempt_count_notrace(val) __preempt_count_sub(val)
#define inc_preempt_count_notrace() add_preempt_count_notrace(1)
#define dec_preempt_count_notrace() sub_preempt_count_notrace(1)