Re: [PATCH v3 1/2] eal: get/set thread priority per thread identifier

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Tue, May 24, 2022 at 07:51:05AM -0700, Stephen Hemminger wrote:
> On Tue, 24 May 2022 04:08:36 -0700
> Tyler Retzlaff <roretzla@linux.microsoft.com> wrote:
> 
> > +static int
> > +thread_map_priority_to_os_value(enum rte_thread_priority eal_pri,
> > +		int *os_pri, int *pol)
> > +{
> > +	/* Clear the output parameters */
> > +	*os_pri = sched_get_priority_min(SCHED_OTHER) - 1;
> > +	*pol = -1;
> > +
> > +	switch (eal_pri) {
> > +	case RTE_THREAD_PRIORITY_NORMAL:
> > +		*pol = SCHED_OTHER;
> > +
> > +		/*
> > +		 * Choose the middle of the range to represent
> > +		 * the priority 'normal'.
> > +		 * On Linux, this should be 0, since both
> > +		 * sched_get_priority_min/_max return 0 for SCHED_OTHER.
> > +		 */
> > +		*os_pri = (sched_get_priority_min(SCHED_OTHER) +
> > +			sched_get_priority_max(SCHED_OTHER))/2;
> > +		break;
> > +	case RTE_THREAD_PRIORITY_REALTIME_CRITICAL:
> > +		*pol = SCHED_RR;
> > +		*os_pri = sched_get_priority_max(SCHED_RR);
> > +		break;
> 
> Many people have experimented with realtime priorities with DPDK
> and Linux, and have never heard of any one not having problems.
> 
> Recommend that this either be an error add a warning log message
> that "Setting real time priority may break"

so i went back through the feedback when the priority change was
originally introduced. you're request was that you didn't want it to be
possible for realtime priority to be set on linux.

this particular function is just about mapping the abstracted
representation of realtime priority in eal to the platform specific
linux representation.

so if you take a look at the other parts of the patch you will notice
that the rte_thread_set_priority() for linux will in fact fail if you
try to use realtime priority.

additionally, the unit test exercises that attempting to use realtime
priority does fail for linux.

take another look and let me know if you don't agree.

thanks