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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/