[PATCH v4.16-rc4 1/2] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW
From: Jason Vas Dias
Date: Mon Mar 12 2018 - 01:01:25 EST
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-rc5 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/include/asm/msr.h
arch/x86/entry/vdso/vclock_gettime.c
arch/x86/entry/vsyscall/vsyscall_gtod.c
This is a resend of the original patch fixing issues
identified by tglx in mail thread of $subject -
mainly that the rdtscp() assembler wrapper function
should be in msr.h - it now is.
There is a second patch following in a few minutes
which adds a record of the calibrated tsc frequency to the VDSO,
and a new header:
uapi/asm/vdso_tsc_calibration.h
which defines a structure :
struct linux_tsc_calibration { u32 tsc_khz, mult, shift ; };
and a getter function in the VDSO that can optionally be used
by user-space code to implement sub-nanosecond precision clocks .
This second patch is entirely optional but I think greatly
expands the scope of user-space TSC readers .
Best Regards,
Jason Vas Dias .
---
diff -up linux-4.16-rc5/arch/x86/entry/vdso/vclock_gettime.c.4.16-rc5 linux-4.16-rc5/arch/x86/entry/vdso/vclock_gettime.c
--- linux-4.16-rc5/arch/x86/entry/vdso/vclock_gettime.c.4.16-rc5 2018-03-12 00:25:09.000000000 +0000
+++ linux-4.16-rc5/arch/x86/entry/vdso/vclock_gettime.c 2018-03-12 04:29:27.296982872 +0000
@@ -182,6 +182,19 @@ 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 ) )
+ tsc = rdtscp((void*)0);
+ 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 +216,27 @@ 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 +280,27 @@ 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 +332,10 @@ 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-rc5/arch/x86/entry/vsyscall/vsyscall_gtod.c.4.16-rc5 linux-4.16-rc5/arch/x86/entry/vsyscall/vsyscall_gtod.c
--- linux-4.16-rc5/arch/x86/entry/vsyscall/vsyscall_gtod.c.4.16-rc5 2018-03-12 00:25:09.000000000 +0000
+++ linux-4.16-rc5/arch/x86/entry/vsyscall/vsyscall_gtod.c 2018-03-12 04:23:10.005141993 +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-rc5/arch/x86/include/asm/msr.h.4.16-rc5 linux-4.16-rc5/arch/x86/include/asm/msr.h
--- linux-4.16-rc5/arch/x86/include/asm/msr.h.4.16-rc5 2018-03-12 00:25:09.000000000 +0000
+++ linux-4.16-rc5/arch/x86/include/asm/msr.h 2018-03-12 04:39:50.415312760 +0000
@@ -218,6 +218,36 @@ static __always_inline unsigned long lon
return rdtsc();
}
+/**
+ * rdtscp() - read the current TSC and (optionally) CPU number, with built-in
+ * cancellation point replacing barrier - only available
+ * if static_cpu_has(X86_FEATURE_RDTSCP) .
+ * returns: The 64-bit Time Stamp Counter (TSC) value.
+ * Optionally, 'cpu_out' can be non-null, and on return it will contain
+ * the number (Intel CPU ID) of the CPU that the task is currently running on.
+ * As does EAX_EDT_RET, this uses the "open-coded asm" style to
+ * force the compiler + assembler to always use (eax, edx, ecx) registers,
+ * NOT whole (rax, rdx, rcx) on x86_64 , because only 32-bit
+ * variables are used - exactly the same code should be generated
+ * for this instruction on 32-bit as on 64-bit when this asm stanza is used.
+ * See: SDM , Vol #2, RDTSCP instruction.
+ */
+static __always_inline u64 rdtscp(u32 *cpu_out)
+{
+ u32 tsc_lo, tsc_hi, tsc_cpu;
+ asm volatile
+ ( "rdtscp"
+ : "=a" (tsc_lo)
+ , "=d" (tsc_hi)
+ , "=c" (tsc_cpu)
+ );
+ if ( unlikely(cpu_out != ((void*)0)) )
+ *cpu_out = tsc_cpu;
+ return ((((u64)tsc_hi) << 32) |
+ (((u64)tsc_lo) & 0x0ffffffffULL )
+ );
+}
+
/* Deprecated, keep it for a cycle for easier merging: */
#define rdtscll(now) do { (now) = rdtsc_ordered(); } while (0)
diff -up linux-4.16-rc5/arch/x86/include/asm/vgtod.h.4.16-rc5 linux-4.16-rc5/arch/x86/include/asm/vgtod.h
--- linux-4.16-rc5/arch/x86/include/asm/vgtod.h.4.16-rc5 2018-03-12 00:25:09.000000000 +0000
+++ linux-4.16-rc5/arch/x86/include/asm/vgtod.h 2018-03-12 04:23:10.006142006 +0000
@@ -22,6 +22,11 @@ struct vsyscall_gtod_data {
u64 mask;
u32 mult;
u32 shift;
+ u64 raw_cycle_last;
+ u64 raw_mask;
+ u32 raw_mult;
+ u32 raw_shift;
+ u32 has_rdtscp;
/* open coded 'struct timespec' */
u64 wall_time_snsec;
@@ -32,6 +37,8 @@ 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;
---