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Date: Wed, 29 Aug 2018 01:36:19 +1000
From: Matthew Rickard <matt@...trans.com.au>
To: Andy Lutomirski <luto@...nel.org>
Cc: Stephen Boyd <sboyd@...nel.org>,
John Stultz <john.stultz@...aro.org>, X86 ML <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO
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@...trans.com.au> 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@...trans.com.au>
>> ---
>> 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;
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