clock_gettime(CLOCK_MONOTONIC, &tp) is very precise on ia64 as it uses
ITC (similar to rdtsc on x86). It's not quite a hrtimer as it is a few
times slower than 1ns. Usually 2-3ns.
clock_getres(CLOCK_MONOTONIC, &res) never reflected that fact and
reported 0.04s precision (1/HZ value).
In https://bugs.gentoo.org/596382 gstreamer's test suite failed loudly
when it noticed precision discrepancy.
Before the change:
clock_getres(CLOCK_MONOTONIC, &res) reported 250Hz precision.
After the change:
clock_getres(CLOCK_MONOTONIC, &res) reports ITC (400Mhz) precision.
The patch is based on matoro's fix. I added a bit of explanation why we
need to special-case arch-specific clock_getres().
CC: Émeric Maschino <emeric.maschino@xxxxxxxxx>
CC: linux-ia64@xxxxxxxxxxxxxxx
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: matoro <matoro_mailinglist_kernel@xxxxxxxxx>
Signed-off-by: Sergei Trofimovich <slyich@xxxxxxxxx>
---
Change since v2:
- Moved matoro to CC
Change since v1:
- No code or in-code comment change
- CCed Émeric
- Added matoro's S-O-B the way matoro comfortable with
- Fixed Subject typo s/clock_getre/clock_getres/
arch/ia64/kernel/sys_ia64.c | 26 ++++++++++++++++++++++++++
arch/ia64/kernel/syscalls/syscall.tbl | 2 +-
2 files changed, 27 insertions(+), 1 deletion(-)
diff --git a/arch/ia64/kernel/sys_ia64.c b/arch/ia64/kernel/sys_ia64.c
index e14db25146c2..d5d47eb4608e 100644
--- a/arch/ia64/kernel/sys_ia64.c
+++ b/arch/ia64/kernel/sys_ia64.c
@@ -166,3 +166,29 @@ ia64_mremap (unsigned long addr, unsigned long old_len, unsigned long new_len, u
force_successful_syscall_return();
return addr;
}
+
+asmlinkage long
+ia64_clock_getres(const clockid_t which_clock, struct __kernel_timespec __user *tp)
+{
+ /*
+ * ia64's clock_gettime() syscall is implemented as a vdso call
+ * fsys_clock_gettime(). Currently it handles only
+ * CLOCK_REALTIME and CLOCK_MONOTONIC. Both are based on
+ * 'ar.itc' counter which gets incremented at a constant
+ * frequency. It's usually 400MHz, ~2.5x times slower than CPU
+ * clock frequency. Which is almost a 1ns hrtimer, but not quite.
+ *
+ * Let's special-case these timers to report correct precision
+ * based on ITC frequency and not HZ frequency for supported
+ * clocks.
+ */
+ switch (which_clock) {
+ case CLOCK_REALTIME:
+ case CLOCK_MONOTONIC:
+ s64 tick_ns = DIV_ROUND_UP(NSEC_PER_SEC, local_cpu_data->itc_freq);
+ struct timespec64 rtn_tp = ns_to_timespec64(tick_ns);
+ return put_timespec64(&rtn_tp, tp);
+ }
+
+ return sys_clock_getres(which_clock, tp);
+}
diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index 78b1d03e86e1..72c929d9902b 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl
@@ -240,7 +240,7 @@
228 common timer_delete sys_timer_delete
229 common clock_settime sys_clock_settime
230 common clock_gettime sys_clock_gettime
-231 common clock_getres sys_clock_getres
+231 common clock_getres ia64_clock_getres
232 common clock_nanosleep sys_clock_nanosleep
233 common fstatfs64 sys_fstatfs64
234 common statfs64 sys_statfs64
--
2.37.2
|