[PATCH v4.16-rc4 1/1] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW

Jason Vas Dias
2018-03-11T19:23:56Z

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, 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 ; when code needs this, the latencies 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 @ 24ns
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-rc4 tree,
current HEAD of :
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
.

The patch affects only files:

arch/x86/include/asm/vgtod.h
arch/x86/entry/vdso/vclock_gettime.c
arch/x86/entry/vsyscall/vsyscall_gtod.c

This is a resend of the original patch fixing indentation issues
after installation of emacs Lisp cc-mode hooks in
Documentation/coding-style.rst
and calling 'indent-region' and 'tabify' (whitespace only changes) - SORRY !

Best Regards,
Jason Vas Dias .

---
diff -up linux-4.16-rc4/arch/x86/entry/vdso/vclock_gettime.c.4.16-rc4 linux-4.16-rc4/arch/x86/entry/vdso/vclock_gettime.c
--- linux-4.16-rc4/arch/x86/entry/vdso/vclock_gettime.c.4.16-rc4 2018-03-04 22:54:11.000000000 +0000
+++ linux-4.16-rc4/arch/x86/entry/vdso/vclock_gettime.c 2018-03-11 19:00:04.630019100 +0000
@@ -182,6 +182,29 @@ notrace static u64 vread_tsc(void)
return last;
}

+notrace static u64 vread_tsc_raw(void)
+{
+ u64 tsc, last=gtod->raw_cycle_last;
+ if( likely( gtod->has_rdtscp ) ) {
+ u32 tsc_lo, tsc_hi,
+ tsc_cpu __attribute__((unused));
+ asm volatile
+ ( "rdtscp"
+ /* ^- has built-in cancellation point / pipeline stall"barrier" */
+ : "=a" (tsc_lo)
+ , "=d" (tsc_hi)
+ , "=c" (tsc_cpu)
+ ); // since all variables 32-bit, eax, edx, ecx used - NOT rax, rdx, rcx
+ tsc = ((((u64)tsc_hi) & 0xffffffffUL) << 32) | (((u64)tsc_lo) & 0xffffffffUL);
+ } else {
+ tsc = rdtsc_ordered();
+ }
+ if (likely(tsc >= last))
+ return tsc;
+ asm volatile ("");
+ return last;
+}
+
notrace static inline u64 vgetsns(int *mode)
{
u64 v;
@@ -203,6 +226,27 @@ notrace static inline u64 vgetsns(int *m
return v * gtod->mult;
}

+notrace static inline u64 vgetsns_raw(int *mode)
+{
+ u64 v;
+ cycles_t cycles;
+
+ if (gtod->vclock_mode == VCLOCK_TSC)
+ cycles = vread_tsc_raw();
+#ifdef CONFIG_PARAVIRT_CLOCK
+ else if (gtod->vclock_mode == VCLOCK_PVCLOCK)
+ cycles = vread_pvclock(mode);
+#endif
+#ifdef CONFIG_HYPERV_TSCPAGE
+ else if (gtod->vclock_mode == VCLOCK_HVCLOCK)
+ cycles = vread_hvclock(mode);
+#endif
+ else
+ return 0;
+ v = (cycles - gtod->raw_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 +290,27 @@ notrace static int __always_inline do_mo
return mode;
}

+notrace static int __always_inline 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 +342,10 @@ notrace int __vdso_clock_gettime(clockid
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 -up linux-4.16-rc4/arch/x86/entry/vsyscall/vsyscall_gtod.c.4.16-rc4 linux-4.16-rc4/arch/x86/entry/vsyscall/vsyscall_gtod.c
--- linux-4.16-rc4/arch/x86/entry/vsyscall/vsyscall_gtod.c.4.16-rc4 2018-03-04 22:54:11.000000000 +0000
+++ linux-4.16-rc4/arch/x86/entry/vsyscall/vsyscall_gtod.c 2018-03-11 19:06:36.584178867 +0000
@@ -16,6 +16,7 @@
#include <linux/timekeeper_internal.h>
#include <asm/vgtod.h>
#include <asm/vvar.h>
+#include <asm/cpufeature.h>

int vclocks_used __read_mostly;

@@ -45,6 +46,12 @@ void update_vsyscall(struct timekeeper *
vdata->mult = tk->tkr_mono.mult;
vdata->shift = tk->tkr_mono.shift;

+ vdata->raw_cycle_last = tk->tkr_raw.cycle_last;
+ vdata->raw_mask = tk->tkr_raw.mask;
+ vdata->raw_mult = tk->tkr_raw.mult;
+ vdata->raw_shift = tk->tkr_raw.shift;
+ vdata->has_rdtscp = static_cpu_has(X86_FEATURE_RDTSCP);
+
vdata->wall_time_sec = tk->xtime_sec;
vdata->wall_time_snsec = tk->tkr_mono.xtime_nsec;

@@ -74,5 +81,8 @@ void update_vsyscall(struct timekeeper *
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 -up linux-4.16-rc4/arch/x86/include/asm/vgtod.h.4.16-rc4 linux-4.16-rc4/arch/x86/include/asm/vgtod.h
--- linux-4.16-rc4/arch/x86/include/asm/vgtod.h.4.16-rc4 2018-03-04 22:54:11.000000000 +0000
+++ linux-4.16-rc4/arch/x86/include/asm/vgtod.h 2018-03-11 18:58:36.670840250 +0000
@@ -22,6 +22,11 @@ struct vsyscall_gtod_data {