Re: [PATCH tip/core/rcu] Reduce overhead of cond_resched() checks for RCU

[Paul E. McKenney]

On Mon, Jun 23, 2014 at 08:26:15AM +0200, Peter Zijlstra wrote:
> On Fri, Jun 20, 2014 at 07:59:58PM -0700, Paul E. McKenney wrote:
> > Commit ac1bea85781e (Make cond_resched() report RCU quiescent states)
> > fixed a problem where a CPU looping in the kernel with but one runnable
> > task would give RCU CPU stall warnings, even if the in-kernel loop
> > contained cond_resched() calls.  Unfortunately, in so doing, it introduced
> > performance regressions in Anton Blanchard's will-it-scale "open1" test.
> > The problem appears to be not so much the increased cond_resched() path
> > length as an increase in the rate at which grace periods complete, which
> > increased per-update grace-period overhead.
> > 
> > This commit takes a different approach to fixing this bug, mainly by
> > moving the RCU-visible quiescent state from cond_resched() to
> > rcu_note_context_switch(), and by further reducing the check to a
> > simple non-zero test of a single per-CPU variable.  However, this
> > approach requires that the force-quiescent-state processing send
> > resched IPIs to the offending CPUs.  These will be sent only once
> > the grace period has reached an age specified by the boot/sysfs
> > parameter rcutree.jiffies_till_sched_qs, or once the grace period
> > reaches an age halfway to the point at which RCU CPU stall warnings
> > will be emitted, whichever comes first.
> 
> Right, and I suppose the force quiescent stuff is triggered from the
> tick, which in turn wakes some of these rcu kthreads, which on UP would
> cause scheduling themselves.

Yep, which is another reason why this commit only affects TREE_RCU and
TREE_PREEMPT_RCU, not TINY_RCU.

> On the topic of these threads; I recently noticed RCU grew a metric ton
> of them, I found some 75 rcu kthreads on my box, wth up with that?

The most likely cause of a recent increase would be if you now have
CONFIG_RCU_NOCB_CPU_ALL=y, which would give you a pair of kthreads per
CPU for callback offloading.  Plus an additional kthread per CPU (for
a total of three new kthreads per CPU) for CONFIG_PREEMPT=y.  These would
be the rcuo kthreads.

Are they causing you trouble?

							Thanx, Paul

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