Current vDSO64 implementation does not have support for coarse clocks
(CLOCK_MONOTONIC_COARSE, CLOCK_REALTIME_COARSE), for which it falls back
to system call, increasing the response time, vDSO implementation reduces
the cycle time. Below is a benchmark of the difference in execution time
with and without vDSO support.
(Non-coarse clocks are also included just for completion)
Without vDSO support:
--------------------
clock-gettime-realtime: syscall: 172 nsec/call
clock-gettime-realtime: libc: 26 nsec/call
clock-gettime-realtime: vdso: 21 nsec/call
clock-gettime-monotonic: syscall: 170 nsec/call
clock-gettime-monotonic: libc: 30 nsec/call
clock-gettime-monotonic: vdso: 24 nsec/call
clock-gettime-realtime-coarse: syscall: 153 nsec/call
clock-gettime-realtime-coarse: libc: 15 nsec/call
clock-gettime-realtime-coarse: vdso: 9 nsec/call
clock-gettime-monotonic-coarse: syscall: 167 nsec/call
clock-gettime-monotonic-coarse: libc: 15 nsec/call
clock-gettime-monotonic-coarse: vdso: 11 nsec/call
CC: Benjamin Herrenschmidt <b...@kernel.crashing.org>
Signed-off-by: Santosh Sivaraj <sant...@fossix.org>
---
arch/powerpc/kernel/asm-offsets.c | 2 ++
arch/powerpc/kernel/vdso64/gettimeofday.S | 56 +++++++++++++++++++++++++++++++
2 files changed, 58 insertions(+)
diff --git a/arch/powerpc/kernel/asm-offsets.c
b/arch/powerpc/kernel/asm-offsets.c
index 8cfb20e38cfe..b55c68c54dc1 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -396,6 +396,8 @@ int main(void)
/* Other bits used by the vdso */
DEFINE(CLOCK_REALTIME, CLOCK_REALTIME);
DEFINE(CLOCK_MONOTONIC, CLOCK_MONOTONIC);
+ DEFINE(CLOCK_REALTIME_COARSE, CLOCK_REALTIME_COARSE);
+ DEFINE(CLOCK_MONOTONIC_COARSE, CLOCK_MONOTONIC_COARSE);
DEFINE(NSEC_PER_SEC, NSEC_PER_SEC);
DEFINE(CLOCK_REALTIME_RES, MONOTONIC_RES_NSEC);
diff --git a/arch/powerpc/kernel/vdso64/gettimeofday.S
b/arch/powerpc/kernel/vdso64/gettimeofday.S
index a0b4943811db..bae197a81add 100644
--- a/arch/powerpc/kernel/vdso64/gettimeofday.S
+++ b/arch/powerpc/kernel/vdso64/gettimeofday.S
@@ -71,6 +71,11 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
cror cr0*4+eq,cr0*4+eq,cr1*4+eq
beq cr0,49f
+ cmpwi cr0,r3,CLOCK_REALTIME_COARSE
+ cmpwi cr1,r3,CLOCK_MONOTONIC_COARSE
+ cror cr0*4+eq,cr0*4+eq,cr1*4+eq
+ beq cr0,65f
+
b 99f /* Fallback to syscall */
.cfi_register lr,r12
49: bl V_LOCAL_FUNC(__get_datapage) /* get data page */
@@ -112,6 +117,57 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
1: bge cr1,80f
addi r4,r4,-1
add r5,r5,r7
+ b 80f
+
+ /*
+ * For coarse clocks we get data directly from the vdso data page, so
+ * we don't need to call __do_get_tspec, but we still need to do the
+ * counter trick.
+ */
+65: bl V_LOCAL_FUNC(__get_datapage) /* get data page */
+70: ld r8,CFG_TB_UPDATE_COUNT(r3)
+ andi. r0,r8,1 /* pending update ? loop */
+ bne- 70b
+ xor r0,r8,r8 /* create dependency */
+ add r3,r3,r0
+
+ /*
+ * CLOCK_REALTIME_COARSE, below values are needed for MONOTONIC_COARSE
+ * too
+ */
+ ld r4,STAMP_XTIME+TSPC64_TV_SEC(r3)
+ ld r5,STAMP_XTIME+TSPC64_TV_NSEC(r3)
+ bne cr1,78f
+
+ /* CLOCK_MONOTONIC_COARSE */
+ lwa r6,WTOM_CLOCK_SEC(r3)
+ lwa r9,WTOM_CLOCK_NSEC(r3)
+
+ /* check if counter has updated */
+78: or r0,r6,r9
+ xor r0,r0,r0
+ add r3,r3,r0
+ ld r0,CFG_TB_UPDATE_COUNT(r3)
+ cmpld cr0,r0,r8 /* check if updated */
+ bne- 70b
+
+ /* Counter has not updated, so continue calculating proper values for
+ * sec and nsec if monotonic coarse, or just return with the proper
+ * values for realtime.
+ */
+ bne cr1,80f
+
+ /* Add wall->monotonic offset and check for overflow or underflow */
+ add r4,r4,r6
+ add r5,r5,r9
+ cmpd cr0,r5,r7
+ cmpdi cr1,r5,0
+ blt 79f
+ subf r5,r7,r5
+ addi r4,r4,1
+79: bge cr1,80f
+ addi r4,r4,-1
+ add r5,r5,r7
80: std r4,TSPC64_TV_SEC(r11)
std r5,TSPC64_TV_NSEC(r11)
--
2.13.5
