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

Peter Zijlstra
2013-08-14T17:31:31Z

On Wed, Aug 14, 2013 at 06:04:16PM +0200, Peter Zijlstra wrote:
> 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.

OK, another hatchet job, now with bits borrowed from Andi's
voluntary-preempt v1. This one actually boots.

---
--- a/arch/x86/include/asm/calling.h
+++ b/arch/x86/include/asm/calling.h
@@ -48,6 +48,8 @@ For 32-bit we have the following convent

#include <asm/dwarf2.h>

+#ifdef CONFIG_X86_64
+
/*
* 64-bit system call stack frame layout defines and helpers,
* for assembly code:
@@ -192,3 +194,51 @@ For 32-bit we have the following convent
.macro icebp
.byte 0xf1
.endm
+
+#else /* CONFIG_X86_64 */
+
+/*
+ * For 32bit only simplified versions of SAVE_ALL/RESTORE_ALL. These
+ * are different from the entry_32.S versions in not changing the segment
+ * registers. So only suitable for in kernel use, not when transitioning
+ * from or to user space. The resulting stack frame is not a standard
+ * pt_regs frame. The main use case is calling C code from assembler
+ * when all the registers need to be preserved.
+ */
+
+ .macro SAVE_ALL
+ pushl_cfi %eax
+ CFI_REL_OFFSET eax, 0
+ pushl_cfi %ebp
+ CFI_REL_OFFSET ebp, 0
+ pushl_cfi %edi
+ CFI_REL_OFFSET edi, 0
+ pushl_cfi %esi
+ CFI_REL_OFFSET esi, 0
+ pushl_cfi %edx
+ CFI_REL_OFFSET edx, 0
+ pushl_cfi %ecx
+ CFI_REL_OFFSET ecx, 0
+ pushl_cfi %ebx
+ CFI_REL_OFFSET ebx, 0
+ .endm
+
+ .macro RESTORE_ALL
+ popl_cfi %ebx
+ CFI_RESTORE ebx
+ popl_cfi %ecx
+ CFI_RESTORE ecx
+ popl_cfi %edx
+ CFI_RESTORE edx
+ popl_cfi %esi
+ CFI_RESTORE esi
+ popl_cfi %edi
+ CFI_RESTORE edi
+ popl_cfi %ebp
+ CFI_RESTORE ebp
+ popl_cfi %eax
+ CFI_RESTORE eax
+ .endm
+
+#endif /* CONFIG_X86_64 */
+
--- 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 volatile("addl %1," __percpu_arg(0) \
+ : "+m" (__preempt_count) \
+ : "ri" ((int)x) \
+ : "memory"); \
+} while (0)
+
+#define __preempt_count_sub(x) do { \
+ asm volatile("subl %1," __percpu_arg(0) \
+ : "+m" (__preempt_count) \
+ : "ri" ((int)x) \
+ : "memory"); \
+} while (0)
+
+#define __preempt_enable() do { \
+ asm volatile("\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/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -36,6 +36,8 @@ obj-y += tsc.o io_delay.o rtc.o
obj-y += pci-iommu_table.o
obj-y += resource.o

+obj-$(CONFIG_PREEMPT) += preempt.o
+
obj-y += process.o
obj-y += i387.o xsave.o
obj-y += ptrace.o
--- /dev/null
+++ b/arch/x86/kernel/preempt.S
@@ -0,0 +1,25 @@
+
+#include <linux/linkage.h>
+#include <asm/dwarf2.h>
+#include <asm/asm.h>
+#include <asm/calling.h>
+
+ENTRY(__preempt_schedule)
+ CFI_STARTPROC
+ SAVE_ALL
+ call preempt_schedule
+ RESTORE_ALL
+ ret
+ CFI_ENDPROC
+
+#ifdef CONFIG_CONTEXT_TRACKING
+
+ENTRY(__preempt_schedule_context)
+ CFI_STARTPROC
+ SAVE_ALL
+ call preempt_schedule_context
+ RESTORE_ALL
+ ret
+ CFI_ENDPROC
+
+#endif
--- 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)
@@ -64,7 +64,7 @@ do { \

#ifdef CONFIG_CONTEXT_TRACKING

-void preempt_schedule_context(void);
+asmlinkage void preempt_schedule_context(void);

#define preempt_check_resched_context() \
do { \
@@ -101,17 +101,19 @@ 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)
+#else
+#define preempt_enable() __preempt_enable()
+#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)