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

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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: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
This commit is contained in:
Vincent Chen
2020-06-09 22:14:48 +08:00
committed by Palmer Dabbelt
parent 05589dde64
commit ad5d1122b8
16 changed files with 199 additions and 69 deletions
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8
arch/riscv/kernel/vdso.c
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@@ -11,8 +11,12 @@
#include <linux/slab.h>
#include <linux/binfmts.h>
#include <linux/err.h>
#include <asm/page.h>
#ifdef GENERIC_TIME_VSYSCALL
#include <vdso/datapage.h>
#else
#include <asm/vdso.h>
#endif
extern char vdso_start[], vdso_end[];
@@ -26,7 +30,7 @@ static union {
struct vdso_data data;
u8 page[PAGE_SIZE];
} vdso_data_store __page_aligned_data;
static struct vdso_data *vdso_data = &vdso_data_store.data;
struct vdso_data *vdso_data = &vdso_data_store.data;
static int __init vdso_init(void)
{