The following commit has been merged into the locking/core branch of tip:
Commit-ID: 2d352ec9fcb5d965318e7855b2406a7a14e9ae13
Gitweb: https://git.kernel.org/tip/2d352ec9fcb5d965318e7855b2406a7a14e9ae13
Author: Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate: Fri, 14 Feb 2025 16:07:47 +01:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 21 Feb 2025 15:56:08 +01:00
x86/locking: Use asm_inline for {,try_}cmpxchg{64,128} emulations
According to:
https://gcc.gnu.org/onlinedocs/gcc/Size-of-an-asm.html
the usage of asm pseudo directives in the asm template can confuse
the compiler to wrongly estimate the size of the generated
code.
The ALTERNATIVE macro expands to several asm pseudo directives,
so its usage in {,try_}cmpxchg{64,128} causes instruction length estimate
to fail by an order of magnitude (the specially instrumented compiler
reports the estimated length of these asm templates to be more than 20
instructions long).
This incorrect estimate further causes unoptimal inlining
decisions, unoptimal instruction scheduling and unoptimal code block
alignments for functions that use these locking primitives.
Use asm_inline instead:
https://gcc.gnu.org/pipermail/gcc-patches/2018-December/512349.html
which is a feature that makes GCC pretend some inline assembler code
is tiny (while it would think it is huge), instead of just asm.
For code size estimation, the size of the asm is then taken as
the minimum size of one instruction, ignoring how many instructions
compiler thinks it is.
The effect of this patch on x86_64 target is minor, since 128-bit
functions are rarely used on this target. The code size of the resulting
defconfig object file stays the same:
text data bss dec hex filename
27456612 4638523 814148 32909283 1f627e3 vmlinux-old.o
27456612 4638523 814148 32909283 1f627e3 vmlinux-new.o
but the patch has minor effect on code layout due to the different
scheduling decisions in functions containing changed macros.
There is no effect on the x64_32 target, the code size of the resulting
defconfig object file and the code layout stays the same:
text data bss dec hex filename
18883870 2679275 1707916 23271061 1631695 vmlinux-old.o
18883870 2679275 1707916 23271061 1631695 vmlinux-new.o
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20250214150929.5780-2-ubizjak@xxxxxxxxx
---
arch/x86/include/asm/cmpxchg_32.h | 32 ++++++------
arch/x86/include/asm/percpu.h | 77 ++++++++++++++----------------
2 files changed, 55 insertions(+), 54 deletions(-)
diff --git a/arch/x86/include/asm/cmpxchg_32.h b/arch/x86/include/asm/cmpxchg_32.h
index fd1282a..95b5f99 100644
--- a/arch/x86/include/asm/cmpxchg_32.h
+++ b/arch/x86/include/asm/cmpxchg_32.h
@@ -91,12 +91,14 @@ static __always_inline bool __try_cmpxchg64_local(volatile u64 *ptr, u64 *oldp,
union __u64_halves o = { .full = (_old), }, \
n = { .full = (_new), }; \
\
- asm volatile(ALTERNATIVE(_lock_loc \
- "call cmpxchg8b_emu", \
- _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
- : ALT_OUTPUT_SP("+a" (o.low), "+d" (o.high)) \
- : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr) \
- : "memory"); \
+ asm_inline volatile( \
+ ALTERNATIVE(_lock_loc \
+ "call cmpxchg8b_emu", \
+ _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
+ : ALT_OUTPUT_SP("+a" (o.low), "+d" (o.high)) \
+ : "b" (n.low), "c" (n.high), \
+ [ptr] "S" (_ptr) \
+ : "memory"); \
\
o.full; \
})
@@ -119,14 +121,16 @@ static __always_inline u64 arch_cmpxchg64_local(volatile u64 *ptr, u64 old, u64
n = { .full = (_new), }; \
bool ret; \
\
- asm volatile(ALTERNATIVE(_lock_loc \
- "call cmpxchg8b_emu", \
- _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
- CC_SET(e) \
- : ALT_OUTPUT_SP(CC_OUT(e) (ret), \
- "+a" (o.low), "+d" (o.high)) \
- : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr) \
- : "memory"); \
+ asm_inline volatile( \
+ ALTERNATIVE(_lock_loc \
+ "call cmpxchg8b_emu", \
+ _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
+ CC_SET(e) \
+ : ALT_OUTPUT_SP(CC_OUT(e) (ret), \
+ "+a" (o.low), "+d" (o.high)) \
+ : "b" (n.low), "c" (n.high), \
+ [ptr] "S" (_ptr) \
+ : "memory"); \
\
if (unlikely(!ret)) \
*(_oldp) = o.full; \
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 0ab991f..08f5f61 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -348,15 +348,14 @@ do { \
old__.var = _oval; \
new__.var = _nval; \
\
- asm qual (ALTERNATIVE("call this_cpu_cmpxchg8b_emu", \
- "cmpxchg8b " __percpu_arg([var]), X86_FEATURE_CX8) \
- : ALT_OUTPUT_SP([var] "+m" (__my_cpu_var(_var)), \
- "+a" (old__.low), \
- "+d" (old__.high)) \
- : "b" (new__.low), \
- "c" (new__.high), \
- "S" (&(_var)) \
- : "memory"); \
+ asm_inline qual ( \
+ ALTERNATIVE("call this_cpu_cmpxchg8b_emu", \
+ "cmpxchg8b " __percpu_arg([var]), X86_FEATURE_CX8) \
+ : ALT_OUTPUT_SP([var] "+m" (__my_cpu_var(_var)), \
+ "+a" (old__.low), "+d" (old__.high)) \
+ : "b" (new__.low), "c" (new__.high), \
+ "S" (&(_var)) \
+ : "memory"); \
\
old__.var; \
})
@@ -378,17 +377,16 @@ do { \
old__.var = *_oval; \
new__.var = _nval; \
\
- asm qual (ALTERNATIVE("call this_cpu_cmpxchg8b_emu", \
- "cmpxchg8b " __percpu_arg([var]), X86_FEATURE_CX8) \
- CC_SET(z) \
- : ALT_OUTPUT_SP(CC_OUT(z) (success), \
- [var] "+m" (__my_cpu_var(_var)), \
- "+a" (old__.low), \
- "+d" (old__.high)) \
- : "b" (new__.low), \
- "c" (new__.high), \
- "S" (&(_var)) \
- : "memory"); \
+ asm_inline qual ( \
+ ALTERNATIVE("call this_cpu_cmpxchg8b_emu", \
+ "cmpxchg8b " __percpu_arg([var]), X86_FEATURE_CX8) \
+ CC_SET(z) \
+ : ALT_OUTPUT_SP(CC_OUT(z) (success), \
+ [var] "+m" (__my_cpu_var(_var)), \
+ "+a" (old__.low), "+d" (old__.high)) \
+ : "b" (new__.low), "c" (new__.high), \
+ "S" (&(_var)) \
+ : "memory"); \
if (unlikely(!success)) \
*_oval = old__.var; \
\
@@ -419,15 +417,14 @@ do { \
old__.var = _oval; \
new__.var = _nval; \
\
- asm qual (ALTERNATIVE("call this_cpu_cmpxchg16b_emu", \
- "cmpxchg16b " __percpu_arg([var]), X86_FEATURE_CX16) \
- : ALT_OUTPUT_SP([var] "+m" (__my_cpu_var(_var)), \
- "+a" (old__.low), \
- "+d" (old__.high)) \
- : "b" (new__.low), \
- "c" (new__.high), \
- "S" (&(_var)) \
- : "memory"); \
+ asm_inline qual ( \
+ ALTERNATIVE("call this_cpu_cmpxchg16b_emu", \
+ "cmpxchg16b " __percpu_arg([var]), X86_FEATURE_CX16) \
+ : ALT_OUTPUT_SP([var] "+m" (__my_cpu_var(_var)), \
+ "+a" (old__.low), "+d" (old__.high)) \
+ : "b" (new__.low), "c" (new__.high), \
+ "S" (&(_var)) \
+ : "memory"); \
\
old__.var; \
})
@@ -449,19 +446,19 @@ do { \
old__.var = *_oval; \
new__.var = _nval; \
\
- asm qual (ALTERNATIVE("call this_cpu_cmpxchg16b_emu", \
- "cmpxchg16b " __percpu_arg([var]), X86_FEATURE_CX16) \
- CC_SET(z) \
- : ALT_OUTPUT_SP(CC_OUT(z) (success), \
- [var] "+m" (__my_cpu_var(_var)), \
- "+a" (old__.low), \
- "+d" (old__.high)) \
- : "b" (new__.low), \
- "c" (new__.high), \
- "S" (&(_var)) \
- : "memory"); \
+ asm_inline qual ( \
+ ALTERNATIVE("call this_cpu_cmpxchg16b_emu", \
+ "cmpxchg16b " __percpu_arg([var]), X86_FEATURE_CX16) \
+ CC_SET(z) \
+ : ALT_OUTPUT_SP(CC_OUT(z) (success), \
+ [var] "+m" (__my_cpu_var(_var)), \
+ "+a" (old__.low), "+d" (old__.high)) \
+ : "b" (new__.low), "c" (new__.high), \
+ "S" (&(_var)) \
+ : "memory"); \
if (unlikely(!success)) \
*_oval = old__.var; \
+ \
likely(success); \
})
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