Is Thread.Priority ignored on Linux?

[packaman]

I'm running a netcoreapp3.1\linux-arm build of my application on a fairly vanilla Debian platform. I want one thread running at higher priority than the others but setting Thread.Priority=Highest doesn't seem to be having any effect. I can see (using 'top -H') that all the native threads are running at normal priority and this is consistent with the behaviour of my application.

I've tried running as root and setcap 'cap_sys_nice=+ep' to no effect.

Am I missing something or is this just not going to work?

Thanks,

Martin

[huoyaoyuan]

Thread.Priority is converted to posix priority here:

runtime/src/coreclr/pal/src/thread/thread.cpp

Lines 1042 to 1221 in 072cbae

	PAL_ERROR
	CorUnix::InternalSetThreadPriority(
	CPalThread *pThread,
	HANDLE hTargetThread,
	int iNewPriority
	)
	{
	PAL_ERROR palError = NO_ERROR;
	CPalThread *pTargetThread = NULL;
	IPalObject *pobjThread = NULL;
	int st;
	int policy;
	struct sched_param schedParam;
	int max_priority;
	int min_priority;
	float posix_priority;
	palError = InternalGetThreadDataFromHandle(
	pThread,
	hTargetThread,
	&pTargetThread,
	&pobjThread
	);
	if (NO_ERROR != palError)
	{
	goto InternalSetThreadPriorityExit;
	}
	pTargetThread->Lock(pThread);
	/* validate the requested priority */
	switch (iNewPriority)
	{
	case THREAD_PRIORITY_TIME_CRITICAL: /* fall through */
	case THREAD_PRIORITY_IDLE:
	break;
	case THREAD_PRIORITY_HIGHEST: /* fall through */
	case THREAD_PRIORITY_ABOVE_NORMAL: /* fall through */
	case THREAD_PRIORITY_NORMAL: /* fall through */
	case THREAD_PRIORITY_BELOW_NORMAL: /* fall through */
	case THREAD_PRIORITY_LOWEST:
	#if PAL_IGNORE_NORMAL_THREAD_PRIORITY
	/* We aren't going to set the thread priority. Just record what it is,
	and exit */
	pTargetThread->m_iThreadPriority = iNewPriority;
	goto InternalSetThreadPriorityExit;
	#endif
	break;
	default:
	ASSERT("Priority %d not supported\n", iNewPriority);
	palError = ERROR_INVALID_PARAMETER;
	goto InternalSetThreadPriorityExit;
	}
	/* check if the thread is still running */
	if (TS_DONE == pTargetThread->synchronizationInfo.GetThreadState())
	{
	/* the thread has exited, set the priority in the thread structure
	and exit */
	pTargetThread->m_iThreadPriority = iNewPriority;
	goto InternalSetThreadPriorityExit;
	}
	/* get the previous thread schedule parameters. We need to know the
	scheduling policy to determine the priority range */
	if (pthread_getschedparam(
	pTargetThread->GetPThreadSelf(),
	&policy,
	&schedParam
	) != 0)
	{
	ASSERT("Unable to get current thread scheduling information\n");
	palError = ERROR_INTERNAL_ERROR;
	goto InternalSetThreadPriorityExit;
	}
	#if !HAVE_SCHED_OTHER_ASSIGNABLE
	/* Defining thread priority for SCHED_OTHER is implementation defined.
	Some platforms like NetBSD cannot reassign it as they are dynamic.
	*/
	if (policy == SCHED_OTHER)
	{
	TRACE("Pthread priority levels for SCHED_OTHER cannot be reassigned on this platform\n");
	goto InternalSetThreadPriorityExit;
	}
	#endif
	#if HAVE_SCHED_GET_PRIORITY
	max_priority = sched_get_priority_max(policy);
	min_priority = sched_get_priority_min(policy);
	if( -1 == max_priority || -1 == min_priority)
	{
	ASSERT("sched_get_priority_min/max failed; error is %d (%s)\n",
	errno, strerror(errno));
	palError = ERROR_INTERNAL_ERROR;
	goto InternalSetThreadPriorityExit;
	}
	#else
	max_priority = PAL_THREAD_PRIORITY_MAX;
	min_priority = PAL_THREAD_PRIORITY_MIN;
	#endif
	TRACE("Pthread priorities for policy %d must be in the range %d to %d\n",
	policy, min_priority, max_priority);
	/* explanation for fancy maths below :
	POSIX doesn't specify the range of thread priorities that can be used
	with pthread_setschedparam. Instead, one must use sched_get_priority_min
	and sched_get_priority_max to obtain the lower and upper bounds of this
	range. Since the PAL also uses a range of values (from Idle [-15] to
	Time Critical [+15]), we have to do a mapping from a known range to an
	unknown (at compilation) range.
	We do this by :
	-substracting the minimal PAL priority from the desired priority. this
	gives a value between 0 and the PAL priority range
	-dividing this value by the PAL priority range. this allows us to
	express the desired priority as a floating-point value between 0 and 1
	-multiplying this value by the PTHREAD priority range. This gives a
	value between 0 and the PTHREAD priority range
	-adding the minimal PTHREAD priority range. This will give us a value
	between the minimal and maximla pthread priority, which should be
	equivalent to the original PAL value.
	example : suppose a pthread range 100 to 200, and a desired priority
	of 0 (halfway between PAL minimum and maximum)
	0 - (IDLE [-15]) = 15
	15 / (TIMECRITICAL[15] - IDLE[-15]) = 0.5
	0.5 * (pthreadmax[200]-pthreadmin[100]) = 50
	50 + pthreadmin[100] = 150 -> halfway between pthread min and max
	*/
	posix_priority = (iNewPriority - THREAD_PRIORITY_IDLE);
	posix_priority /= (THREAD_PRIORITY_TIME_CRITICAL - THREAD_PRIORITY_IDLE);
	posix_priority *= (max_priority-min_priority);
	posix_priority += min_priority;
	schedParam.sched_priority = (int)posix_priority;
	TRACE("PAL priority %d is mapped to pthread priority %d\n",
	iNewPriority, schedParam.sched_priority);
	/* Finally, set the new priority into place */
	st = pthread_setschedparam(pTargetThread->GetPThreadSelf(), policy, &schedParam);
	if (st != 0)
	{
	#if SET_SCHEDPARAM_NEEDS_PRIVS
	if (EPERM == st)
	{
	// UNIXTODO: Should log a warning to the event log
	TRACE("Caller does not have OS privileges to call pthread_setschedparam\n");
	pTargetThread->m_iThreadPriority = iNewPriority;
	goto InternalSetThreadPriorityExit;
	}
	#endif
	ASSERT("Unable to set thread priority to %d (error %d)\n", (int)posix_priority, st);
	palError = ERROR_INTERNAL_ERROR;
	goto InternalSetThreadPriorityExit;
	}
	pTargetThread->m_iThreadPriority = iNewPriority;
	InternalSetThreadPriorityExit:
	if (NULL != pTargetThread)
	{
	pTargetThread->Unlock(pThread);
	}
	if (NULL != pobjThread)
	{
	pobjThread->ReleaseReference(pThread);
	}
	return palError;
	}

[packaman]

Thanks. So it looks like it should work but I can't get it to work for me. I've posted a follow-up on Stack Overflow