linux-kernel - Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer

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Message-Id: <1612838134.rvncv9kzls.astroid@bobo.none>
Date:   Tue, 09 Feb 2021 12:36:20 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>, msuchanek@...e.de,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in
 another pointer

Excerpts from Christophe Leroy's message of February 9, 2021 1:10 am:
> By saving the pointer pointing to thread_info.flags, gcc copies r2
> in a non-volatile register.
> 
> We know 'current' doesn't change, so avoid that intermediaite pointer.
> 
> Reduces null_syscall benchmark by 2 cycles (322 => 320 cycles)
> 
> On PPC64, gcc seems to know that 'current' is not changing, and it keeps
> it in a non volatile register to avoid multiple read of 'current' in paca.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

What if you did this?

---
 arch/powerpc/include/asm/current.h | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/current.h b/arch/powerpc/include/asm/current.h
index bbfb94800415..59ab327972a5 100644
--- a/arch/powerpc/include/asm/current.h
+++ b/arch/powerpc/include/asm/current.h
@@ -23,16 +23,19 @@ static inline struct task_struct *get_current(void)
 
 	return task;
 }
-#define current	get_current()
 
 #else
 
-/*
- * We keep `current' in r2 for speed.
- */
-register struct task_struct *current asm ("r2");
+static inline struct task_struct *get_current(void)
+{
+	register struct task_struct *task asm ("r2");
+
+	return task;
+}
 
 #endif
 
+#define current	get_current()
+
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_CURRENT_H */
--