[PATCH v4.16-rc6 (1)] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall

[PATCH v4.16-rc6 (1)] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall

[jason.vas.dias]

  Resent to address reviewer comments, and allow builds with compilers
  that support -DRETPOLINE to succeed.
  
  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-rc6 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.
  
  Please consider applying something like this patch to a future Linux release.

  This patch is being resent because it has slight improvements to vclock_gettime
  static function attributes wrt. the previous version.

  It also supersedes all previous patches with subject matching
     '.*VDSO should handle.*clock_gettime.*MONOTONIC_RAW'
  that I have sent previously - sorry for the resends.

  Please apply this patch so we stop getting emails from
  intel build bot trying to build previous version, with
  subject :
    '[PATCH v4.16-rc5 1/2] x86/vdso: VDSO should handle \
     clock_gettime(CLOCK_MONOTONIC_RAW) without syscall'
  , which only fails to build because its patch 2/2 , which
  removed -DRETPOLINE from the VDSO build, and is now the
  subject of https://bugzilla.kernel.org/show_bug.cgi?id=199129,
  raised by H.J. Liu, was not applied first - Sorry! 

Thanks & Best Regards,
Jason Vas Dias

[PATCH v4.16-rc6 1/1] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall

[jason.vas.dias]

    This patch implements clock_gettime(CLOCK_MONOTONIC_RAW,&ts) calls entirely in the
    vDSO, without calling vdso_fallback_gettime() .

    It has been augmented to support compilation with or without -DRETPOLINE / $(RETPOLINE_CFLAGS) ;
    when compiled with -DRETPOLINE, not all functions calls can be inlined within __vdso_clock_gettime,
    and all functions invoked by __vdso_clock_gettime must have 'indirect_branch("keep")' +
    'function_return("keep")' attributes to compile, otherwise thunk relocations will be generated ;
    and the functions cannot all be declared '__always_inline_', otherwise a compiler -Werror
    ('not all __always_inline__ functions can be inlined')  is generated.
    Also, compared to previous version of same patch,  the do_*_coarse functions are still
    not inlines, and not inadvertently changed to inline.

    I still think it might be better to apply H.J. Liu's patch from
    https://bugzilla.kernel.org/show_bug.cgi?id=199129 to disable
    -DRETPOLINE compilation for the vDSO .

---
diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index f19856d..80d65d4 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -182,29 +182,62 @@ notrace static u64 vread_tsc(void)
 	return last;
 }
 
-notrace static inline u64 vgetsns(int *mode)
+notrace static 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->mask;
+	return v * gtod->raw_mult;
+}
+
+#ifdef	RETPOLINE
+#	define	_NO_THUNK_RELOCS_()(indirect_branch("keep"),\
+				    function_return("keep"))
+#	define	_RETPOLINE_FUNC_ATTR_	__attribute__(_NO_THUNK_RELOCS_())
+#	define	_RETPOLINE_INLINE_	inline
+#else
+#	define	_RETPOLINE_FUNC_ATTR_
+#	define	_RETPOLINE_INLINE_	__always_inline
+#endif
+
 /* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
-notrace static int __always_inline do_realtime(struct timespec *ts)
+notrace static _RETPOLINE_INLINE_ _RETPOLINE_FUNC_ATTR_
+int do_realtime(struct timespec *ts)
 {
 	unsigned long seq;
 	u64 ns;
@@ -225,7 +258,8 @@ notrace static int __always_inline do_realtime(struct timespec *ts)
 	return mode;
 }
 
-notrace static int __always_inline do_monotonic(struct timespec *ts)
+notrace static _RETPOLINE_INLINE_ _RETPOLINE_FUNC_ATTR_
+int do_monotonic(struct timespec *ts)
 {
 	unsigned long seq;
 	u64 ns;
@@ -246,7 +280,30 @@ notrace static int __always_inline do_monotonic(struct timespec *ts)
 	return mode;
 }
 
-notrace static void do_realtime_coarse(struct timespec *ts)
+notrace static _RETPOLINE_INLINE_ _RETPOLINE_FUNC_ATTR_
+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 _RETPOLINE_FUNC_ATTR_
+void do_realtime_coarse(struct timespec *ts)
 {
 	unsigned long seq;
 	do {
@@ -256,7 +313,8 @@ notrace static void do_realtime_coarse(struct timespec *ts)
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 }
 
-notrace static void do_monotonic_coarse(struct timespec *ts)
+notrace static _RETPOLINE_FUNC_ATTR_
+void do_monotonic_coarse(struct timespec *ts)
 {
 	unsigned long seq;
 	do {
@@ -266,7 +324,8 @@ notrace static void do_monotonic_coarse(struct timespec *ts)
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 }
 
-notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
+notrace _RETPOLINE_FUNC_ATTR_
+int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 {
 	switch (clock) {
 	case CLOCK_REALTIME:
@@ -277,6 +336,10 @@ 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..c4d89b6 100644
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -44,6 +44,8 @@ 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;
 
 	vdata->wall_time_sec		= tk->xtime_sec;
 	vdata->wall_time_snsec		= tk->tkr_mono.xtime_nsec;
@@ -74,5 +76,8 @@ 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..ec1a37c 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -22,7 +22,8 @@ struct vsyscall_gtod_data {
 	u64	mask;
 	u32	mult;
 	u32	shift;
-
+	u32	raw_mult;
+	u32	raw_shift;
 	/* open coded 'struct timespec' */
 	u64		wall_time_snsec;
 	gtod_long_t	wall_time_sec;
@@ -32,6 +33,8 @@ 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;

Re: [PATCH v4.16-rc6 1/1] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall

[Jason Vas Dias]

Good day -

I believe the last patch I sent, with $subject,
addresses all concerns raised so far by reviewers,
and complies with all kernel coding standards .

Please, it would be most helpful if you could let
me know whether the patch is now acceptable
and will be applied at some stage or not - or if not,
what is the problem with it .

My clients are asking whether the patch is going
to be in the upstream kernel or not, and I need
to tell them something.

Thanks & Best Regards,
Jason

[PATCH v4.16-rc6 (1)] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall

[jason.vas.dias]

Resent to address reviewer comments, and allow builds with compilers
  that support -DRETPOLINE to succeed.

  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.

  Please consider applying something like this patch to a future Linux release.

Thanks & Best Regards,
Jason Vas Dias

Re: [PATCH v4.16-rc6 (1)] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall

[Jason Vas Dias]

Note there is a bug raised by H.J. Liu :
     Bug 199129: Don't build vDSO with $(RETPOLINE_CFLAGS) -DRETPOLINE
            (https://bugzilla.kernel.org/show_bug.cgi?id=199129)

If you agree it is a bug, then use both patches from post :
    '[PATCH v4.16-rc5 (2)] x86/vdso: VDSO should handle \
     clock_gettime(CLOCK_MONOTONIC_RAW) without syscall
    '
else, use the single patch from $subject, which makes the
calls to the statics in vclock_gettime.c' use
   indirect_branch("keep") / function_return("keep") ,
to avoid generation of thunk relocations which would not
occur unless compiled with
   -mindirect-branch=thunk-extern -mindirect-branch-register
.

Thanks & Regards,
Jason