Re: [PATCH v6 1/1] eal/unix: allow creating thread with real-time priority

[Thomas Monjalon <thomas@monjalon.net>]

27/10/2023 10:45, Morten Brørup:
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > Sent: Friday, 27 October 2023 10.09
> > 
> > When adding an API for creating threads,
> > the real-time priority has been forbidden on Unix.
> > 
> > There is a known issue with ring behaviour,
> > but it should not be completely forbidden.
> > 
> > Real-time thread can block some kernel threads on the same core,
> > making the system unstable.
> > That's why a sleep is added in the test thread,
> > and a warning is logged when using real-time priority.
> > 
> > Fixes: ca04c78b6262 ("eal: get/set thread priority per thread
> > identifier")
> > Fixes: ce6e911d20f6 ("eal: add thread lifetime API")
> > Fixes: a7ba40b2b1bf ("drivers: convert to internal control threads")
> > Cc: stable@dpdk.org
> > 
> > Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
> > Acked-by: Morten Brørup <mb@smartsharesystems.com>
> > ---
> 
> [...]
> 
> >  enum rte_thread_priority {
> > +	/** Normal thread priority, the default. */
> >  	RTE_THREAD_PRIORITY_NORMAL            = 0,
> > -	/**< normal thread priority, the default */
> > +	/**
> > +	 * Highest thread priority, use with caution.
> > +	 * WARNING: System may be unstable because of a real-time busy
> > loop.
> > +	 *          @see rte_thread_yield_realtime().
> 
> Please remove the reference to the now non-existing function.
> 
> Also, I'd prefer to move the warning comments (about real-time threads having priority over kernel threads, and issues with rte_ring) up here, so it goes into the public API documentation.

Yes OK, thanks for the careful review.

> 
> > +	 */
> >  	RTE_THREAD_PRIORITY_REALTIME_CRITICAL = 1,
> > -	/**< highest thread priority allowed */
> >  };
> > 
> >  /**
> > diff --git a/lib/eal/unix/rte_thread.c b/lib/eal/unix/rte_thread.c
> > index 278d8d342d..17ffb86c17 100644
> > --- a/lib/eal/unix/rte_thread.c
> > +++ b/lib/eal/unix/rte_thread.c
> > @@ -33,6 +33,8 @@ static int
> >  thread_map_priority_to_os_value(enum rte_thread_priority eal_pri, int
> > *os_pri,
> >  	int *pol)
> >  {
> > +	static bool warned;
> > +
> >  	/* Clear the output parameters. */
> >  	*os_pri = sched_get_priority_min(SCHED_OTHER) - 1;
> >  	*pol = -1;
> > @@ -51,6 +53,17 @@ thread_map_priority_to_os_value(enum
> > rte_thread_priority eal_pri, int *os_pri,
> >  			sched_get_priority_max(SCHED_OTHER)) / 2;
> >  		break;
> >  	case RTE_THREAD_PRIORITY_REALTIME_CRITICAL:
> > +		/*
> > +		 * WARNING: Real-time busy loop takes priority on kernel
> > threads,
> > +		 *          making the system unstable.
> > +		 *          There is also a known issue when using
> > rte_ring.
> > +		 */
> > +		if (!warned) {
> > +			RTE_LOG(NOTICE, EAL,
> > +					"Real-time thread is unstable if polling
> > without sleep.\n");
> > +			warned = true;
> > +		}
> 
> Is it 100 % certain that the system becomes unstable if not sleeping or using blocking system calls from a real-time thread?
> And technically, it's not the thread itself that becomes unstable.
> 
> How about:
> "System may be unstable unless real-time thread uses blocking system calls or sleeps."
> or:
> "Real-time thread usually requires the use of blocking system calls or sleeps."
> or something else.

Yes something like that looks better.
I will try to find a short sentence.

> 
> My ACK is still valid.
> 
>