linux-kernel - [PATCH v2] x86/vdso: implement clock_gettime(CLOCK_MONOTONIC

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Date:   Tue, 4 Jun 2019 16:42:20 +0000
From:   "Sverdlin, Alexander (Nokia - DE/Ulm)" <alexander.sverdlin@...ia.com>
To:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Sverdlin, Alexander (Nokia - DE/Ulm)" <alexander.sverdlin@...ia.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Vas Dias <jason.vas.dias@...il.com>
Subject: [PATCH v2] x86/vdso: implement clock_gettime(CLOCK_MONOTONIC_RAW,
 ...)

From: Alexander Sverdlin <alexander.sverdlin@...ia.com>

Add CLOCK_MONOTONIC_RAW to the existing clock_gettime() vDSO
implementation. This is based on the ideas of Jason Vas Dias and comments
of Thomas Gleixner.

---- Test code ----
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
 #include <unistd.h>

 #define CLOCK_TYPE CLOCK_MONOTONIC_RAW
 #define DURATION_SEC 10

int main(int argc, char **argv)
{
	struct timespec t, end;
	unsigned long long cnt = 0;

	clock_gettime(CLOCK_TYPE, &end);
	end.tv_sec += DURATION_SEC;

	do {
		clock_gettime(CLOCK_TYPE, &t);
		++cnt;
	} while (t.tv_sec < end.tv_sec || t.tv_nsec < end.tv_nsec);

	dprintf(STDOUT_FILENO, "%llu", cnt);

	return EXIT_SUCCESS;
}
-------------------

The results from the above test program:

Clock                   Before  After   Diff
-----                   ------  -----   ----
CLOCK_MONOTONIC         355.5M  355.5M
CLOCK_MONOTONIC_RAW      44.9M  371.2M  +726%
CLOCK_REALTIME          355.5M  355.5M

Link: https://lore.kernel.org/patchwork/patch/933583/
Link: https://bugzilla.kernel.org/show_bug.cgi?id=198961
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Jason Vas Dias <jason.vas.dias@...il.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...ia.com>
---
Changelog:
v2: copy do_hres() into do_monotonic_raw()

 arch/x86/entry/vdso/vclock_gettime.c    | 35 +++++++++++++++++++++++++++++++++
 arch/x86/entry/vsyscall/vsyscall_gtod.c |  6 ++++++
 arch/x86/include/asm/vgtod.h            |  2 ++
 3 files changed, 43 insertions(+)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 0f82a70..64736a4 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -168,6 +168,39 @@ notrace static int do_hres(clockid_t clk, struct timespec *ts)
 	return 0;
 }
 
+/*
+ * Attempts to merge the below copy with the above routine led to 5% performance
+ * drop (CLOCK_MONOTONIC, CLOCK_REALTIME) up to now. Test before making changes.
+ */
+notrace static int do_monotonic_raw(struct timespec *ts)
+{
+	struct vgtod_ts *base = &gtod->basetime[CLOCK_MONOTONIC_RAW];
+	u64 cycles, last, sec, ns;
+	unsigned int seq;
+
+	do {
+		seq = gtod_read_begin(gtod);
+		cycles = vgetcyc(gtod->vclock_mode);
+		ns = base->nsec;
+		last = gtod->cycle_last;
+		if (unlikely((s64)cycles < 0))
+			return vdso_fallback_gettime(CLOCK_MONOTONIC_RAW, ts);
+		if (cycles > last)
+			ns += (cycles - last) * gtod->raw_mult;
+		ns >>= gtod->raw_shift;
+		sec = base->sec;
+	} while (unlikely(gtod_read_retry(gtod, seq)));
+
+	/*
+	 * Do this outside the loop: a race inside the loop could result
+	 * in __iter_div_u64_rem() being extremely slow.
+	 */
+	ts->tv_sec = sec + __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
+	ts->tv_nsec = ns;
+
+	return 0;
+}
+
 notrace static void do_coarse(clockid_t clk, struct timespec *ts)
 {
 	struct vgtod_ts *base = &gtod->basetime[clk];
@@ -199,6 +232,8 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 		do_coarse(clock, ts);
 		return 0;
 	}
+	if (clock == CLOCK_MONOTONIC_RAW)
+		return do_monotonic_raw(ts);
 	return vdso_fallback_gettime(clock, ts);
 }
 
diff --git a/arch/x86/entry/vsyscall/vsyscall_gtod.c b/arch/x86/entry/vsyscall/vsyscall_gtod.c
index cfcdba0..9f7744f3 100644
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -46,6 +46,8 @@ 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_mult		= tk->tkr_raw.mult;
+	vdata->raw_shift	= tk->tkr_raw.shift;
 
 	base = &vdata->basetime[CLOCK_REALTIME];
 	base->sec = tk->xtime_sec;
@@ -65,6 +67,10 @@ void update_vsyscall(struct timekeeper *tk)
 	}
 	base->nsec = nsec;
 
+	base = &vdata->basetime[CLOCK_MONOTONIC_RAW];
+	base->sec = tk->raw_sec;
+	base->nsec = tk->tkr_raw.xtime_nsec;
+
 	base = &vdata->basetime[CLOCK_REALTIME_COARSE];
 	base->sec = tk->xtime_sec;
 	base->nsec = tk->tkr_mono.xtime_nsec >> tk->tkr_mono.shift;
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index 913a133..65ac320 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -43,6 +43,8 @@ struct vsyscall_gtod_data {
 	u64		mask;
 	u32		mult;
 	u32		shift;
+	u32		raw_mult;
+	u32		raw_shift;
 
 	struct vgtod_ts	basetime[VGTOD_BASES];
 
-- 
2.4.6