Resent to address reviewer comments.
Currently, the VDSO does not handle
clock_gettime( CLOCK_MONOTONIC_RAW, &ts )
on Intel / AMD - it calls
vdso_fallback_gettime()
for this clock, which issues a syscall, having an unacceptably high
latency (minimum measurable time or time between measurements)
of 300-700ns on 2 2.8-3.9ghz Haswell x86_64 Family'_'Model : 06_3C
machines under various versions of Linux.
Sometimes, particularly when correlating elapsed time to performance
counter values, user-space code needs to know elapsed time from the
perspective of the CPU no matter how "hot" / fast or "cold" / slow it
might be running wrt NTP / PTP "real" time; when code needs this,
the latencies associated with a syscall are often unacceptably high.
I reported this as Bug #198161 :
'https://bugzilla.kernel.org/show_bug.cgi?id=198961'
and in previous posts with subjects matching 'CLOCK_MONOTONIC_RAW' .
This patch handles CLOCK_MONOTONIC_RAW clock_gettime() in the VDSO ,
by exporting the raw clock calibration, last cycles, last xtime_nsec,
and last raw_sec value in the vsyscall_gtod_data during vsyscall_update() .
Now the new do_monotonic_raw() function in the vDSO has a latency of @ 20ns
on average, and the test program:
tools/testing/selftest/timers/inconsistency-check.c
succeeds with arguments: '-c 4 -t 120' or any arbitrary -t value.
The patch is against Linus' latest 4.16-rc5 tree,
current HEAD of :
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
.
This patch affects only files:
arch/x86/include/asm/vgtod.h
arch/x86/entry/vdso/vclock_gettime.c
arch/x86/entry/vsyscall/vsyscall_gtod.c
Patches for kernels 3.10.0-21 and 4.9.65-rt23 (ARM) are attached to bug #198161,
as is the test program, timer_latency.c, to demonstrate the problem.
Before the patch a latency of 200-1000ns was measured for
clock_gettime(CLOCK_MONOTONIC_RAW,&ts)
calls - after the patch, the same call on the same machine
has a latency of @ 20ns.
Thanks & Best Regards,
Jason Vas Dias
diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index f19856d..8b9b9cf 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -182,27 +182,49 @@ notrace static u64 vread_tsc(void)
return last;
}
-notrace static inline u64 vgetsns(int *mode)
+notrace static inline __always_inline u64 vgetcycles(int *mode)
{
- u64 v;
- cycles_t cycles;
-
- if (gtod->vclock_mode == VCLOCK_TSC)
- cycles = vread_tsc();
+ switch (gtod->vclock_mode) {
+ case VCLOCK_TSC:
+ return vread_tsc();
#ifdef CONFIG_PARAVIRT_CLOCK
- else if (gtod->vclock_mode == VCLOCK_PVCLOCK)
- cycles = vread_pvclock(mode);
+ case VCLOCK_PVCLOCK:
+ return vread_pvclock(mode);
#endif
#ifdef CONFIG_HYPERV_TSCPAGE
- else if (gtod->vclock_mode == VCLOCK_HVCLOCK)
- cycles = vread_hvclock(mode);
+ case VCLOCK_HVCLOCK:
+ return vread_hvclock(mode);
#endif
- else
+ default:
+ break;
+ }
+ return 0;
+}
+
+notrace static inline u64 vgetsns(int *mode)
+{
+ u64 v;
+ cycles_t cycles = vgetcycles(mode);
+
+ if (cycles == 0)
return 0;
+
v = (cycles - gtod->cycle_last) & gtod->mask;
return v * gtod->mult;
}
+notrace static inline u64 vgetsns_raw(int *mode)
+{
+ u64 v;
+ cycles_t cycles = vgetcycles(mode);
+
+ if (cycles == 0)
+ return 0;
+
+ v = (cycles - gtod->cycle_last) & gtod->raw_mask;
+ return v * gtod->raw_mult;
+}
+
/* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
notrace static int __always_inline do_realtime(struct timespec *ts)
{
@@ -246,6 +268,27 @@ notrace static int __always_inline do_monotonic(struct timespec *ts)
return mode;
}
+notrace static __always_inline int do_monotonic_raw(struct timespec *ts)
+{
+ unsigned long seq;
+ u64 ns;
+ int mode;
+
+ do {
+ seq = gtod_read_begin(gtod);
+ mode = gtod->vclock_mode;
+ ts->tv_sec = gtod->monotonic_time_raw_sec;
+ ns = gtod->monotonic_time_raw_nsec;
+ ns += vgetsns_raw(&mode);
+ ns >>= gtod->raw_shift;
+ } while (unlikely(gtod_read_retry(gtod, seq)));
+
+ ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
+ ts->tv_nsec = ns;
+
+ return mode;
+}
+
notrace static void do_realtime_coarse(struct timespec *ts)
{
unsigned long seq;
@@ -277,6 +320,10 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
if (do_monotonic(ts) == VCLOCK_NONE)
goto fallback;
break;
+ case CLOCK_MONOTONIC_RAW:
+ if (do_monotonic_raw(ts) == VCLOCK_NONE)
+ goto fallback;
+ break;
case CLOCK_REALTIME_COARSE:
do_realtime_coarse(ts);
break;
diff --git a/arch/x86/entry/vsyscall/vsyscall_gtod.c b/arch/x86/entry/vsyscall/vsyscall_gtod.c
index e1216dd..83f5c21 100644
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -44,6 +44,9 @@ void update_vsyscall(struct timekeeper *tk)
vdata->mask = tk->tkr_mono.mask;
vdata->mult = tk->tkr_mono.mult;
vdata->shift = tk->tkr_mono.shift;
+ vdata->raw_mask = tk->tkr_raw.mask;
+ vdata->raw_mult = tk->tkr_raw.mult;
+ vdata->raw_shift = tk->tkr_raw.shift;
vdata->wall_time_sec = tk->xtime_sec;
vdata->wall_time_snsec = tk->tkr_mono.xtime_nsec;
@@ -74,5 +77,8 @@ void update_vsyscall(struct timekeeper *tk)
vdata->monotonic_time_coarse_sec++;
}
+ vdata->monotonic_time_raw_sec = tk->raw_sec;
+ vdata->monotonic_time_raw_nsec = tk->tkr_raw.xtime_nsec;
+
gtod_write_end(vdata);
}
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index fb856c9..941e9d6 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -22,7 +22,9 @@ struct vsyscall_gtod_data {
u64 mask;
u32 mult;
u32 shift;
-
+ u32 raw_mask;
+ u32 raw_mult;
+ u32 raw_shift;
/* open coded 'struct timespec' */
u64 wall_time_snsec;
gtod_long_t wall_time_sec;
@@ -32,6 +34,8 @@ struct vsyscall_gtod_data {
gtod_long_t wall_time_coarse_nsec;
gtod_long_t monotonic_time_coarse_sec;
gtod_long_t monotonic_time_coarse_nsec;
+ gtod_long_t monotonic_time_raw_sec;
+ gtod_long_t monotonic_time_raw_nsec;
int tz_minuteswest;
int tz_dsttime;
Hi,
Thank you for the patch! Yet something to improve:
[auto build test ERROR on v4.16-rc4]
[also build test ERROR on next-20180316]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
url: https://github.com/0day-ci/linux/commits/jason-vas-dias-gmail-com/x86-vdso-VDSO-should-handle-clock_gettime-CLOCK_MONOTONIC_RAW-without-syscall/20180317-101131
config: x86_64-kexec (attached as .config)
compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
reproduce:
# save the attached .config to linux build tree
make ARCH=x86_64
All errors (new ones prefixed by >>):
arch/x86//entry/vdso/vclock_gettime.o: In function `__vdso_clock_gettime':
vclock_gettime.c:(.text+0xe6): undefined reference to `__x86_indirect_thunk_rax'
/usr/bin/ld: arch/x86//entry/vdso/vclock_gettime.o: relocation R_X86_64_PC32 against undefined symbol `__x86_indirect_thunk_rax' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
>> collect2: error: ld returned 1 exit status
--
>> objcopy: 'arch/x86//entry/vdso/vdso64.so.dbg': No such file
--
>> arch/x86//entry/vdso/vdso32.so.dbg: undefined symbols found
--
arch/x86/entry/vdso/vclock_gettime.o: In function `__vdso_clock_gettime':
vclock_gettime.c:(.text+0xe6): undefined reference to `__x86_indirect_thunk_rax'
/usr/bin/ld: arch/x86/entry/vdso/vclock_gettime.o: relocation R_X86_64_PC32 against undefined symbol `__x86_indirect_thunk_rax' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
>> collect2: error: ld returned 1 exit status
---
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