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[148.163.158.5]) by mx.google.com with ESMTPS id g144si13150913wmg.12.2016.12.01.10.42.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Dec 2016 10:42:57 -0800 (PST) Received: from pps.filterd (m0098413.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uB1IcPtZ030316 for ; Thu, 1 Dec 2016 13:42:55 -0500 Received: from e32.co.us.ibm.com (e32.co.us.ibm.com [32.97.110.150]) by mx0b-001b2d01.pphosted.com with ESMTP id 272rfmbaaf-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 01 Dec 2016 13:42:55 -0500 Received: from localhost by e32.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 1 Dec 2016 11:42:54 -0700 Date: Thu, 1 Dec 2016 10:42:52 -0800 From: "Paul E. McKenney" Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` Reply-To: paulmck@linux.vnet.ibm.com References: <20161130142955.GS3924@linux.vnet.ibm.com> <20161130163820.GQ3092@twins.programming.kicks-ass.net> <20161130170557.GK18432@dhcp22.suse.cz> <20161130175015.GR3092@twins.programming.kicks-ass.net> <20161130194019.GF3924@linux.vnet.ibm.com> <20161201053035.GC3092@twins.programming.kicks-ass.net> <20161201124024.GB3924@linux.vnet.ibm.com> <20161201163614.GL3092@twins.programming.kicks-ass.net> <20161201165918.GG3924@linux.vnet.ibm.com> <20161201180953.GO3045@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161201180953.GO3045@worktop.programming.kicks-ass.net> Message-Id: <20161201184252.GP3924@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Michal Hocko , Donald Buczek , Paul Menzel , dvteam@molgen.mpg.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Josh Triplett On Thu, Dec 01, 2016 at 07:09:53PM +0100, Peter Zijlstra wrote: > On Thu, Dec 01, 2016 at 08:59:18AM -0800, Paul E. McKenney wrote: > > On Thu, Dec 01, 2016 at 05:36:14PM +0100, Peter Zijlstra wrote: > > > Well, with the above change cond_resched() is already sufficient, no? > > > > Maybe. Right now, cond_resched_rcu_qs() gets a quiescent state to > > the RCU core in less than one jiffy, with my other change, this becomes > > a handful of jiffies depending on HZ and NR_CPUS. I expect this > > increase to a handful of jiffies to be a non-event. > > > > After my upcoming patch, cond_resched() will get a quiescent state to > > the RCU core in about ten seconds. While I am am not all that nervous > > about the increase from less than a jiffy to a handful of jiffies, > > increasing to ten seconds via cond_resched() does make me quite nervous. > > Past experience indicates that someone's kernel will likely be fatally > > inconvenienced by this magnitude of change. > > > > Or am I misunderstanding what you are proposing? > > No, that is indeed what I was proposing. Hurm.. OK let me ponder that a > bit. There might be a few games we can play with !PREEMPT to avoid IPIs. > > Thing is, I'm slightly uncomfortable with de-coupling rcu-sched from > actual schedule() calls. OK, what is the source of your discomfort? There are several intermediate levels of evasive action: 0. If there is another runnable task and certain other conditions are met, cond_resched() will invoke schedule(), which will provide an RCU quiescent state. 1. All cond_resched_rcu_qs() invocations increment the CPU's rcu_qs_ctr per-CPU variable, which is treated by later invocations of RCU core as a quiescent state. (I have a patch queued that causes RCU to ignore changes to this counter until the grace period is a few jiffies old.) In this case, the rcu_node locks plus smp_mb__after_unlock_lock() provide the needed ordering. 2. If any cond_resched_rcu_qs() sees that an expedited grace period is waiting on the current CPU, it invokes rcu_sched_qs() to force RCU to see the quiescent state. (To your point, rcu_sched_qs() is normally called from schedule(), but also from the scheduling-clock interrupt when it interrupts usermode or idle.) Again, the rcu_node locks plus smp_mb__after_unlock_lock() provide the needed ordering. 3. If the grace period extends for more than 50 milliseconds (by default, tunable), all subsequent cond_resched_rcu_qs() invocations on that CPU turn into momentary periods of idleness from RCU's viewpoint. (Atomically add 2 to the dyntick-idle counter.) Here, the atomic increment is surrounded by smp_mb__*_atomic() to provide the needed ordering, which should be a good substitute for actually passing through schedule(). 4. If the grace period extends for more than 21 seconds (by default), we emit an RCU CPU stall warning and then do a resched_cpu(). I am proposing also doing a resched_cpu() halfway to RCU CPU stall-warning time. 5. An RCU-sched expedited grace period does a local resched_cpu() from its IPI handler to force the CPU through a quiescent state. (Yes, I could just invoke resched_cpu() from the task orchestrating the expedited grace period, but this approach allows more common code between RCU-preempt and RCU-sched expedited grace periods.) > > > In fact, by doing the IPI thing we get the entire cond_resched*() > > > family, and we could add the should_resched() guard to > > > cond_resched_rcu(). > > > > So that cond_resched_rcu_qs() looks something like this, in order > > to avoid the function call in the case where the scheduler has nothing > > to do? > > I was actually thinking of this: Oh! I had forgotten about cond_resched_rcu(), and thought you did a typo. Acked-by: Paul E. McKenney > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 2d0c82e1d348..2dc7d8056b2a 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -3374,9 +3374,11 @@ static inline int signal_pending_state(long state, struct task_struct *p) > static inline void cond_resched_rcu(void) > { > #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU) > - rcu_read_unlock(); > - cond_resched(); > - rcu_read_lock(); > + if (should_resched(1)) { > + rcu_read_unlock(); > + cond_resched(); > + rcu_read_lock(); > + } > #endif > } > > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org