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ia64: fix clock_getres(CLOCK_MONOTONIC) to report ITC frequency

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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().

[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/20220820181813.2275195-1-slyich@gmail.com
Signed-off-by: Sergei Trofimovich <slyich@gmail.com>
Cc: matoro <matoro_mailinglist_kernel@matoro.tk>
Cc: Émeric Maschino <emeric.maschino@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Sergei Trofimovich 2022-08-20 19:18:13 +01:00 committed by Andrew Morton
parent cba7543e15
commit aa06a9bd85
2 changed files with 27 additions and 1 deletions
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26
arch/ia64/kernel/sys_ia64.c
View File

@ -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);
}

2
arch/ia64/kernel/syscalls/syscall.tbl
View File

@ -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