Messages in this thread Patch in this message | | | From | Sergei Trofimovich <> | Subject | [PATCH v2] ia64: fix clock_getres(CLOCK_MONOTONIC) to report ITC frequency | Date | Sat, 20 Aug 2022 19:18:13 +0100 |
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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@gmail.com> CC: linux-ia64@vger.kernel.org CC: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: matoro <matoro_mailinglist_kernel@matoro.tk> Signed-off-by: Sergei Trofimovich <slyich@gmail.com> --- 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 -- 2.37.1
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