linux-kernel - Re: [PATCH tip/core/rcu 4/7] rcu: Unify boost and kthread priorities

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Message-ID: <20141031165143.GN10501@worktop.programming.kicks-ass.net>
Date:	Fri, 31 Oct 2014 17:51:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	Clark Williams <clark.williams@...il.com>
Subject: Re: [PATCH tip/core/rcu 4/7] rcu: Unify boost and kthread priorities

On Fri, Oct 31, 2014 at 09:42:30AM -0700, Paul E. McKenney wrote:

> Well, you are supposed to determine the highest RT priority at which
> your workload might run CPU-bound tasks, and set the boost priority
> at some level above that.  My model of RCU priority boosting is that
> it should be used to make inadvertent high-priority infinite loops
> easier to debug, but others might have different approaches.

Ah, so DL will never be CPU-bound -- and RR/FIFO _should_ never be, but
I digress ;-)

> > We should be able to detect the case where more and work piles on and
> > the actual running does not appear to catch up, but I'm not sure what to
> > do about it, seeing how system stability is at risk.
> 
> I could imagine having a backup SCHED_FIFO task that handled the
> case where callbacks were piling up, but synchronizing it with the
> SCHED_DEADLINE task while avoiding callback misordering could be a bit
> "interesting".  (Recall that callback misordering messes up rcu_barrier().)

Ah, so there is talk of 'soft' CBS modes, which instead of hard throttle
either reclaim 'unused' DL bandwidth, or continue running in lower scheduling
classes.


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