|From:||"Paul E. McKenney" <paulmck-AT-linux.vnet.ibm.com>|
|To:||Peter Zijlstra <peterz-AT-infradead.org>|
|Subject:||Re: [PATCH RFC tip/core/rcu 4/6] rcu: Clarify help text for RCU_BOOST_PRIO|
|Date:||Thu, 26 Apr 2012 10:28:59 -0700|
|Cc:||linux-kernel-AT-vger.kernel.org, mingo-AT-elte.hu, laijs-AT-cn.fujitsu.com, dipankar-AT-in.ibm.com, akpm-AT-linux-foundation.org, mathieu.desnoyers-AT-polymtl.ca, josh-AT-joshtriplett.org, niv-AT-us.ibm.com, tglx-AT-linutronix.de, rostedt-AT-goodmis.org, Valdis.Kletnieks-AT-vt.edu, dhowells-AT-redhat.com, eric.dumazet-AT-gmail.com, darren-AT-dvhart.com, fweisbec-AT-gmail.com, patches-AT-linaro.org, "Paul E. McKenney" <paul.mckenney-AT-linaro.org>|
On Thu, Apr 26, 2012 at 02:46:31PM +0200, Peter Zijlstra wrote: > On Mon, 2012-04-23 at 09:42 -0700, Paul E. McKenney wrote: > > + This option specifies the real-time priority to which long-term > > + preempted RCU readers are to be boosted. If you are working > > + with a real-time application that has one or more CPU-bound > > + threads running at a real-time priority level, > > Then your application is broken ;-) the kernel is known to mis-behave > under these circumstances since it doesn't get to run house-keeping > tasks. RCU is just one of these and elevating it doesn't make it work. As you say, CPU-bound RT tasks have a number of problems, and RCU is but one of them. That said, an RCU-induced memory-exhaustion system hang is an extremely unfriendly diagnostic message, and use of RCU priority boosting allows them a better debugging environment. > > you should set > > + RCU_BOOST_PRIO to a priority higher then the highest-priority > > + real-time CPU-bound thread. The default RCU_BOOST_PRIO value > > + of 1 is appropriate in the common case, which is real-time > > + applications that do not have any CPU-bound threads. > > Alternatively, 1 is the worst possible choice forcing people to consider > the issue. You say that as if forcing people to consider the issue was a bad thing. ;-) > > + Some real-time applications might not have a single real-time > > + thread that saturates a given CPU, but instead might have > > + multiple real-time threads that, taken together, fully utilize > > + that CPU. In this case, you should set RCU_BOOST_PRIO to > > + a priority higher than the lowest-priority thread that is > > + conspiring to prevent the CPU from running any non-real-time > > + tasks. For example, if one thread at priority 10 and another > > + thread at priority 5 are between themselves fully consuming > > + the CPU time on a given CPU, then RCU_BOOST_PRIO should be > > + set to priority 6 or higher. > > I'd call this misleading, who's to say that preempting the 5 would yield > enough time to complete the RCU work? Yep, hence the "or higher". > This all gets us back to the fun question of RCU delayed bandwidth > budgeting.. ideally every 'task' that does call_rcu() should donate some > of its budget towards the thread running the callback. There was an academic interested in that topic a few years ago, but I don't believe anything came of it. An interesting approach would be to do EDF scheduling on the callbacks themselves, but having a separate thread for each callback sounds like overkill. > Anyway, I'd argue both the old and new description are bonkers. Indeed, my goal was "less bonkers" rather than "not bonkers". A "not bonkers" description remains a long-term aspiration rather than a short-term goal for the moment. I can only hope that the timeframe is shorter than it was for RCU back in the early 1990s. ;-) Thanx, Paul
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