From ac0cf0338cd06c0b848f1b59a2192fdada745373 Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Fri, 4 Feb 2022 10:44:39 +0000 Subject: [PATCH] arm64: atomics: remove redundant static branch Due to a historical oversight, we emit a redundant static branch for each atomic/atomic64 operation when CONFIG_ARM64_LSE_ATOMICS is selected. We can safely remove this, making the kernel Image reasonably smaller. When CONFIG_ARM64_LSE_ATOMICS is selected, every LSE atomic operation has two preceding static branches with the same target, e.g. b f7c b f7c mov w0, #0x1 // #1 ldadd w0, w0, [x19] This is because the __lse_ll_sc_body() wrapper uses system_uses_lse_atomics(), which checks both `arm64_const_caps_ready` and `cpu_hwcap_keys[ARM64_HAS_LSE_ATOMICS]`, each of which emits a static branch. This has been the case since commit: addfc38672c73efd ("arm64: atomics: avoid out-of-line ll/sc atomics") However, there was never a need to check `arm64_const_caps_ready`, which was itself introduced in commit: 63a1e1c95e60e798 ("arm64/cpufeature: don't use mutex in bringup path") ... so that cpus_have_const_cap() could fall back to checking the `cpu_hwcaps` bitmap prior to the static keys for individual caps becoming enabled. As system_uses_lse_atomics() doesn't check `cpu_hwcaps`, and doesn't need to as we can safely use the LL/SC atomics prior to enabling the `ARM64_HAS_LSE_ATOMICS` static key, it doesn't need to check `arm64_const_caps_ready`. This patch removes the `arm64_const_caps_ready` check from system_uses_lse_atomics(). As the arch_atomic_* routines are meant to be safely usable in noinstr code, I've also marked system_uses_lse_atomics() as __always_inline. This results in one fewer static branch per atomic operation, with the prior example becoming: b f78 mov w0, #0x1 // #1 ldadd w0, w0, [x19] Each static branch consists of the branch itself and an associated __jump_table entry. Removing these has a reasonable impact on the Image size, with a GCC 11.1.0 defconfig v5.17-rc2 Image being reduced by 128KiB: | [mark@lakrids:~/src/linux]% ls -al Image* | -rw-r--r-- 1 mark mark 34619904 Feb 3 18:24 Image.baseline | -rw-r--r-- 1 mark mark 34488832 Feb 3 18:33 Image.onebranch Signed-off-by: Mark Rutland Cc: Ard Biesheuvel Cc: Catalin Marinas Cc: Suzuki Poulose Cc: Will Deacon Link: https://lore.kernel.org/r/20220204104439.270567-1-mark.rutland@arm.com Signed-off-by: Will Deacon --- arch/arm64/include/asm/lse.h | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/arch/arm64/include/asm/lse.h b/arch/arm64/include/asm/lse.h index 5de132100b6d..d492a5caba0c 100644 --- a/arch/arm64/include/asm/lse.h +++ b/arch/arm64/include/asm/lse.h @@ -17,12 +17,10 @@ #include extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS]; -extern struct static_key_false arm64_const_caps_ready; -static inline bool system_uses_lse_atomics(void) +static __always_inline bool system_uses_lse_atomics(void) { - return (static_branch_likely(&arm64_const_caps_ready)) && - static_branch_likely(&cpu_hwcap_keys[ARM64_HAS_LSE_ATOMICS]); + return static_branch_likely(&cpu_hwcap_keys[ARM64_HAS_LSE_ATOMICS]); } #define __lse_ll_sc_body(op, ...) \