Re: [RESEND PATCH] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO
From: Matthew Rickard
Date: Tue Aug 28 2018 - 11:36:26 EST
Here are the before and after times with CONFIG_RETPOLINE=y always on.
I don't see any regression, just the hoped-for improvement on glibc and
vDSO calls of CLOCK_TAI.
Before:
sec Timestamp nanos clockname tzname type
---------- --------------------------- ----- --------- --------- -------
1535445844 2018/08/28 08:44:04.338599419 96 CLOCK_REALTIME UTC 0 glibc
1535445844 2018/08/28 08:44:04.348494684 87 CLOCK_REALTIME UTC 1 vdso
1535445844 2018/08/28 08:44:04.357328913 321 CLOCK_REALTIME UTC 2 sys
1535445834 2018/08/28 08:43:27.507099055 233 CLOCK_TAI right/UTC 0 glibc
1535445834 2018/08/28 08:43:27.530666383 239 CLOCK_TAI right/UTC 1 vdso
1535445834 2018/08/28 08:43:27.554827262 389 CLOCK_TAI right/UTC 2 sys
80 1970/01/01 00:01:20.593942210 88 CLOCK_MONOTONIC UTC 0 glibc
80 1970/01/01 00:01:20.602866312 84 CLOCK_MONOTONIC UTC 1 vdso
80 1970/01/01 00:01:20.611322392 272 CLOCK_MONOTONIC UTC 2 sys
80 1970/01/01 00:01:20.638630685 298 CLOCK_BOOTTIME UTC 0 glibc
80 1970/01/01 00:01:20.668487920 293 CLOCK_BOOTTIME UTC 1 vdso
80 1970/01/01 00:01:20.697818847 279 CLOCK_BOOTTIME UTC 2 sys
After yours and my patches:
sec Timestamp nanos clockname tzname type
---------- --------------------------- ----- --------- --------- -------
1535466985 2018/08/28 14:36:25.483377529 93 CLOCK_REALTIME UTC 0 glibc
1535466985 2018/08/28 14:36:25.493020681 89 CLOCK_REALTIME UTC 1 vdso
1535466985 2018/08/28 14:36:25.502139080 282 CLOCK_REALTIME UTC 2 sys
1535466975 2018/08/28 14:35:48.530621935 87 CLOCK_TAI right/UTC 0 glibc
1535466975 2018/08/28 14:35:48.539393751 81 CLOCK_TAI right/UTC 1 vdso
1535466975 2018/08/28 14:35:48.547693183 276 CLOCK_TAI right/UTC 2 sys
224 1970/01/01 00:03:44.575542852 87 CLOCK_MONOTONIC UTC 0 glibc
224 1970/01/01 00:03:44.584329822 81 CLOCK_MONOTONIC UTC 1 vdso
224 1970/01/01 00:03:44.592473982 269 CLOCK_MONOTONIC UTC 2 sys
224 1970/01/01 00:03:44.619450784 296 CLOCK_BOOTTIME UTC 0 glibc
224 1970/01/01 00:03:44.649224430 312 CLOCK_BOOTTIME UTC 1 vdso
224 1970/01/01 00:03:44.680600544 297 CLOCK_BOOTTIME UTC 2 sys
-Matt-
On 25/08/2018 3:47 AM, Andy Lutomirski wrote:
Minor nit: if it's not literally a resend, don't call it "RESEND" in
$SUBJECT. Call it v2, please.
Also, I added LKML and relevant maintainers to cc. John and Stephen:
this is a purely x86 patch, but it digs into the core timekeeping
structures a bit.
On Fri, Aug 17, 2018 at 5:12 AM, Matt Rickard <matt@xxxxxxxxxxxxxxx> wrote:
Process clock_gettime(CLOCK_TAI) in vDSO. This makes the call about as fast as
CLOCK_REALTIME instead of taking about four times as long.
I'm conceptually okay with this, but the bug encountered last time
around makes me suspect that GCC is generating genuinely horrible
code. Can you benchmark CLOCK_MONOTONIC before and after to make sure
there isn't a big regression? Please do this benchmark with
CONFIG_RETPOLINE=y.
If there is a regression, then the code will need some reasonable
restructuring to fix it. Or perhaps -fno-jump-tables.
--Andy
Signed-off-by: Matt Rickard <matt@xxxxxxxxxxxxxxx>
---
arch/x86/entry/vdso/vclock_gettime.c | 25 +++++++++++++++++++++++++
arch/x86/entry/vsyscall/vsyscall_gtod.c | 2 ++
arch/x86/include/asm/vgtod.h | 1 +
3 files changed, 28 insertions(+)
diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index f19856d95c60..91ed1bb2a3bb 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -246,6 +246,27 @@ notrace static int __always_inline do_monotonic(struct timespec *ts)
return mode;
}
+notrace static int __always_inline do_tai(struct timespec *ts)
+{
+ unsigned long seq;
+ u64 ns;
+ int mode;
+
+ do {
+ seq = gtod_read_begin(gtod);
+ mode = gtod->vclock_mode;
+ ts->tv_sec = gtod->tai_time_sec;
+ ns = gtod->wall_time_snsec;
+ ns += vgetsns(&mode);
+ ns >>= gtod->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 +298,10 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
if (do_monotonic(ts) == VCLOCK_NONE)
goto fallback;
break;
+ case CLOCK_TAI:
+ if (do_tai(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 e1216dd95c04..d61392fe17f6 100644
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -53,6 +53,8 @@ void update_vsyscall(struct timekeeper *tk)
vdata->monotonic_time_snsec = tk->tkr_mono.xtime_nsec
+ ((u64)tk->wall_to_monotonic.tv_nsec
<< tk->tkr_mono.shift);
+ vdata->tai_time_sec = tk->xtime_sec
+ + tk->tai_offset;
while (vdata->monotonic_time_snsec >=
(((u64)NSEC_PER_SEC) << tk->tkr_mono.shift)) {
vdata->monotonic_time_snsec -=
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index fb856c9f0449..adc9f7b20b9c 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -32,6 +32,7 @@ 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 tai_time_sec;
int tz_minuteswest;
int tz_dsttime;