RFR: 8372625: [Linux] Remove unnecessary logic for supports_fast_thread_cpu_time

[Jonas Norlinder]

On Thu, 27 Nov 2025 04:04:50 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> For Linux we test if we can use `CLOCK_THREAD_CPUTIME_ID` but not the clocks supported by `pthread_getcpuclockid()`.
>> 
>> The following comment has been around for at least 18 years according to the git commit history (points to the initial git commit from Dec 1, 2007):
>> 
>> 
>> // Switch to using fast clocks for thread cpu time if
>> // the sys_clock_getres() returns 0 error code.
>> // Note, that some kernels may support the current thread
>> // clock (CLOCK_THREAD_CPUTIME_ID) but not the clocks
>> // returned by the pthread_getcpuclockid().
>> // If the fast Posix clocks are supported then the sys_clock_getres()
>> // must return at least tp.tv_sec == 0 which means a resolution
>> // better than 1 sec. This is extra check for reliability.
>> 
>> 
>> There was a time when the Linux kernel did not provide an implementation for `CLOCK_THREAD_CPUTIME_ID` (that arrived with v2.6.12). During this period, glibc emulated support for `clock_gettime(CLOCK_THREAD_CPUTIME_ID)` in userspace for the current thread only. `clock_gettime(CLOCK_THREAD_CPUTIME_ID)` will call `pthread_getcpuclockid` in https://elixir.bootlin.com/glibc/glibc-2.2.5/source/linuxthreads/sysdeps/pthread/getcpuclockid.c. The checks and comment exists because `CLOCK_THREAD_CPUTIME_ID` worked (via emulation) but `pthread_getcpuclockid()` failed. 
>> 
>> During that period of the early 2000s this check was indeed needed. But now sufficient time has passed and we don't support OpenJDK for that old kernel versions/glibc. Therefore, this check should be removed and some code can be cleaned up.
>
> src/hotspot/os/linux/os_linux.cpp line 162:
> 
>> 160: uintptr_t os::Linux::_initial_thread_stack_size   = 0;
>> 161: 
>> 162: int (*os::Linux::_pthread_getcpuclockid)(pthread_t, clockid_t *) = (int(*)(pthread_t, clockid_t *)) dlsym(RTLD_DEFAULT, "pthread_getcpuclockid");
> 
> I expected to see this go away. Are we not guaranteed to have the function available at build time and run time?

Thanks good point!

> src/hotspot/os/linux/os_linux.cpp line 4955:
> 
>> 4953:                  &user_time, &sys_time);
>> 4954:   if (count != 13) return -1;
>> 4955:   return ((jlong)sys_time + (jlong)user_time) * (1000000000 / os::Posix::clock_tics_per_second());
> 
> If this function is meant to be `user` time you kept the wrong line.

It returns the user_time variable. Could you please elaborate why that is wrong?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28516#discussion_r2567349819
PR Review Comment: https://git.openjdk.org/jdk/pull/28516#discussion_r2567345303