From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Tue, 3 Jan 2017 18:27:01 +0000 Subject: [PATCH] arm64: restore get_current() optimisation Message-ID: <1483468021-8237-1-git-send-email-mark.rutland@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Catalin, My THREAD_INFO_IN_TASK series had an unintended performance regression in get_current() / current_thread_info(). Could you please take the below as a fix for the next rc? Thanks, Mark. ---->8---- Commit c02433dd6de32f04 ("arm64: split thread_info from task stack") inverted the relationship between get_current() and current_thread_info(), with sp_el0 now holding the current task_struct rather than the current thead_info. The new implementation of get_current() prevents the compiler from being able to optimize repeated calls to either, resulting in a noticeable penalty in some microbenchmarks. This patch restores the previous optimisation by implementing get_current() in the same way as our old current_thread_info(), using a non-volatile asm statement. Signed-off-by: Mark Rutland Cc: Will Deacon Cc: Catalin Marinas Reported-by: Davidlohr Bueso --- arch/arm64/include/asm/current.h | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/arch/arm64/include/asm/current.h b/arch/arm64/include/asm/current.h index f2bcbe2..86c4041 100644 --- a/arch/arm64/include/asm/current.h +++ b/arch/arm64/include/asm/current.h @@ -9,9 +9,17 @@ struct task_struct; +/* + * We don't use read_sysreg() as we want the compiler to cache the value where + * possible. + */ static __always_inline struct task_struct *get_current(void) { - return (struct task_struct *)read_sysreg(sp_el0); + unsigned long sp_el0; + + asm ("mrs %0, sp_el0" : "=r" (sp_el0)); + + return (struct task_struct *)sp_el0; } #define current get_current() -- 1.9.1