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From: Thomas Monjalon <thomas@monjalon.net>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: dev@dpdk.org, David Marchand <david.marchand@redhat.com>,
	stable@dpdk.org,  Anatoly Burakov <anatoly.burakov@intel.com>,
	Tyler Retzlaff <roretzla@linux.microsoft.com>,
	Narcisa Vasile <navasile@linux.microsoft.com>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>,
	Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>,
	Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Subject: Re: [PATCH v6 1/1] eal/unix: allow creating thread with real-time priority
Date: Fri, 27 Oct 2023 11:11:19 +0200	[thread overview]
Message-ID: <2088214.KlZ2vcFHjT@thomas> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9EFA3@smartserver.smartshare.dk>

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.
> 
> 






  reply	other threads:[~2023-10-27  9:11 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-10-24 12:54 [PATCH] " Thomas Monjalon
2023-10-24 13:55 ` Morten Brørup
2023-10-24 16:04   ` Stephen Hemminger
2023-10-25 13:15     ` Thomas Monjalon
2023-10-25 13:34       ` Bruce Richardson
2023-10-25 13:44         ` Thomas Monjalon
2023-10-25 15:08           ` Stephen Hemminger
2023-10-25 15:14             ` Bruce Richardson
2023-10-25 15:18               ` Thomas Monjalon
2023-10-25 15:32                 ` Thomas Monjalon
2023-10-25 15:13 ` [PATCH v2] " Thomas Monjalon
2023-10-25 15:37   ` Stephen Hemminger
2023-10-25 16:46     ` Thomas Monjalon
2023-10-25 17:54       ` Morten Brørup
2023-10-25 21:33         ` Stephen Hemminger
2023-10-26  7:33           ` Morten Brørup
2023-10-26 16:32             ` Stephen Hemminger
2023-10-26 17:07               ` Morten Brørup
2023-10-26  0:00         ` Stephen Hemminger
     [not found] ` <20231025163352.1076755-1-thomas@monjalon.net>
2023-10-25 16:31   ` [PATCH v3 2/2] " Thomas Monjalon
     [not found] ` <20231026134313.1165954-1-thomas@monjalon.net>
2023-10-26 13:37   ` [PATCH v4 " Thomas Monjalon
     [not found] ` <20231026142749.1174372-1-thomas@monjalon.net>
2023-10-26 14:19   ` [PATCH v5 " Thomas Monjalon
2023-10-27  8:08 ` [PATCH v6 1/1] " Thomas Monjalon
2023-10-27  8:45   ` Morten Brørup
2023-10-27  9:11     ` Thomas Monjalon [this message]
2023-10-27 18:15     ` Stephen Hemminger
2024-10-07 19:27   ` Stephen Hemminger

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