Unverified Commit ad5d1122 authored by Vincent Chen's avatar Vincent Chen Committed by Palmer Dabbelt
Browse files

riscv: use vDSO common flow to reduce the latency of the time-related functions 



Even if RISC-V has supported the vDSO feature, the latency of the functions
for obtaining the system time is still expensive. It is because these
functions still trigger a corresponding system call in the process, which
slows down the response time. If we want to remove the system call to
reduce the latency, the kernel should have the ability to output the system
clock information to userspace. This patch introduces the vDSO common flow
to enable the kernel to achieve the above feature and uses "rdtime"
instruction to obtain the current time in the user space. Under this
condition, the latency cost by the ecall from U-mode to S-mode can be
eliminated. After applying this patch, the latency of gettimeofday()
measured on the HiFive unleashed board can be reduced by %61.

Signed-off-by: default avatarVincent Chen <vincent.chen@sifive.com>
Reviewed-by: default avatarAtish Patra <atish.patra@wdc.com>
Signed-off-by: default avatarPalmer Dabbelt <palmerdabbelt@google.com>
parent 05589dde

arch/riscv/Kconfig

+4 −0
Original line number Diff line number Diff line
@@ -12,6 +12,7 @@ config 32BIT 


config RISCV

	def_bool y

	select ARCH_CLOCKSOURCE_INIT

	select ARCH_HAS_BINFMT_FLAT

	select ARCH_HAS_DEBUG_VIRTUAL if MMU

	select ARCH_HAS_DEBUG_WX
@@ -31,6 +32,7 @@ config RISCV 
	select GENERIC_ARCH_TOPOLOGY if SMP

	select GENERIC_ATOMIC64 if !64BIT

	select GENERIC_CLOCKEVENTS

	select GENERIC_GETTIMEOFDAY if HAVE_GENERIC_VDSO

	select GENERIC_IOREMAP

	select GENERIC_IRQ_MULTI_HANDLER

	select GENERIC_IRQ_SHOW
@@ -40,6 +42,7 @@ config RISCV 
	select GENERIC_SMP_IDLE_THREAD

	select GENERIC_STRNCPY_FROM_USER if MMU

	select GENERIC_STRNLEN_USER if MMU

	select GENERIC_TIME_VSYSCALL if MMU && 64BIT

	select HANDLE_DOMAIN_IRQ

	select HAVE_ARCH_AUDITSYSCALL

	select HAVE_ARCH_KASAN if MMU && 64BIT
@@ -53,6 +56,7 @@ config RISCV 
	select HAVE_DMA_CONTIGUOUS if MMU

	select HAVE_EBPF_JIT if MMU

	select HAVE_FUTEX_CMPXCHG if FUTEX

	select HAVE_GENERIC_VDSO if MMU && 64BIT

	select HAVE_PCI

	select HAVE_PERF_EVENTS

	select HAVE_PERF_REGS

arch/riscv/include/asm/clocksource.h

0 → 100644
+7 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

#ifndef _ASM_CLOCKSOURCE_H

#define _ASM_CLOCKSOURCE_H



#include <asm/vdso/clocksource.h>



#endif

arch/riscv/include/asm/processor.h

+2 −10
Original line number Diff line number Diff line
@@ -8,6 +8,8 @@ 


#include <linux/const.h>



#include <vdso/processor.h>



#include <asm/ptrace.h>



/*
@@ -58,16 +60,6 @@ static inline void release_thread(struct task_struct *dead_task) 
extern unsigned long get_wchan(struct task_struct *p);





static inline void cpu_relax(void)

{

#ifdef __riscv_muldiv

	int dummy;

	/* In lieu of a halt instruction, induce a long-latency stall. */

	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));

#endif

	barrier();

}



static inline void wait_for_interrupt(void)

{

	__asm__ __volatile__ ("wfi");

arch/riscv/include/asm/vdso.h

+2 −0
Original line number Diff line number Diff line
@@ -10,8 +10,10 @@ 


#include <linux/types.h>



#ifndef GENERIC_TIME_VSYSCALL

struct vdso_data {

};

#endif



/*

 * The VDSO symbols are mapped into Linux so we can just use regular symbol

arch/riscv/include/asm/vdso/clocksource.h

0 → 100644
+8 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

#ifndef __ASM_VDSOCLOCKSOURCE_H

#define __ASM_VDSOCLOCKSOURCE_H



#define VDSO_ARCH_CLOCKMODES	\

	VDSO_CLOCKMODE_ARCHTIMER



#endif

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