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Date: Sat, 2 Dec 2017 11:24:19 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Frederic Weisbecker <frederic@...nel.org> Cc: linux-kernel@...r.kernel.org, xiaolong.ye@...el.com, tglx@...utronix.de, cmetcalf@...lanox.com, cl@...ux.com, torvalds@...ux-foundation.org, lcapitulino@...hat.com, efault@....de, peterz@...radead.org, riel@...hat.com, kernellwp@...il.com, mingo@...nel.org, john.stultz@...aro.org Subject: Re: [PATCH] sched/isolation: Make NO_HZ_FULL select CPU_ISOLATION On Sat, Dec 02, 2017 at 02:59:12PM +0100, Frederic Weisbecker wrote: > 2017-11-30 21:20 UTC+01:00, Paul E. McKenney <paulmck@...ux.vnet.ibm.com>: > > Commit 5c4991e24c69 ("sched/isolation: Split out new > > CONFIG_CPU_ISOLATION=y config from CONFIG_NO_HZ_FULL") can result in RCU > > CPU stall warnings when running rcutorture with CONFIG_NO_HZ_FULL_ALL=y > > and CONFIG_CPU_ISOLATION=n. These warnings are caused by RCU's > > grace-period kthreads sleeping for a few jiffies, but never being > > awakened: > > > > [ 116.353432] rcu_preempt kthread starved for 9974 jiffies! g4294967208 > > +c4294967207 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x402 ->cpu=0 > > [ 116.355517] rcu_preempt I 7464 8 2 0x80000000 > > [ 116.356543] Call Trace: > > [ 116.357008] __schedule+0x493/0x620 > > [ 116.357682] schedule+0x24/0x40 > > [ 116.358291] schedule_timeout+0x330/0x3b0 > > [ 116.359024] ? preempt_count_sub+0xea/0x140 > > [ 116.359806] ? collect_expired_timers+0xb0/0xb0 > > [ 116.360660] rcu_gp_kthread+0x6bf/0xef0 > > > > This commit therefore makes NO_HZ_FULL select CPU_ISOLATION, which > > prevents this behavior and seems like it was the original intention in > > any case. > > Although CONFIG_NO_HZ should indeed select CONFIG_CPU_ISOLATION, I'm > surprised about this stall. I'm even more surprised that setting > CONFIG_CPU_ISOLATION=y is enough to fix the issue because > CONFIG_NO_HZ_FULL_ALL shortcuts CONFIG_CPU_ISOLATION entirely (which > is not good, but work in progress...). Yes, and after applying this patch, I get failures a few commits later, which appears to be due to other changes that break CONFIG_NO_HZ_FULL_ALL=y. So I have another patch staged that removes CONFIG_NO_HZ_FULL_ALL, on the grounds that no one else has complained, so rcutorture is likely to be the only user, and I don't see the point of having a Kconfig option for only one user. > Did you have any nohz_full= or isolcpus= boot options? Replacing CONFIG_NO_HZ_FULL_ALL=y with nohz_full=1-7 works, that is CONFIG_NO_HZ_FULL=y, CONFIG_NO_HZ_FULL_ALL=n, and nohz_full=1-7 on an eight-CPU test. But it is relatively easy to test. Running the rcutorture TREE04 scenario on a four-socket x86 gets me RCU CPU stall warnings within a few minutes more than half the time. ;-) Thanx, Paul
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