From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
oleg@redhat.com, Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH tip/core/rcu 06/10] trace: Eliminate cond_resched_rcu_qs() in favor of cond_resched()
Date: Sun, 25 Feb 2018 10:17:30 -0800 [thread overview]
Message-ID: <20180225181730.GA3963@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180225174927.GC2855@linux.vnet.ibm.com>
On Sun, Feb 25, 2018 at 09:49:27AM -0800, Paul E. McKenney wrote:
> On Sat, Feb 24, 2018 at 03:12:40PM -0500, Steven Rostedt wrote:
> > On Fri, 1 Dec 2017 11:21:40 -0800
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > Now that cond_resched() also provides RCU quiescent states when
> > > needed, it can be used in place of cond_resched_rcu_qs(). This
> > > commit therefore makes this change.
> >
> > Are you sure this is true?
>
> Up to a point. If a given CPU has been blocking an RCU grace period for
> long enough, that CPU's rcu_dynticks.rcu_need_heavy_qs will be set, and
> then the next cond_resched() will be treated as a cond_resched_rcu_qs().
>
> However, to your point, if there is no grace period in progress or if
> the current grace period is not waiting on the CPU in question or if
> the grace-period kthread is starved of CPU, then cond_resched() has no
> effect on RCU. Unless of course it results in a context switch.
>
> > I just bisected a lock up on my machine down to this commit.
> >
> > With CONFIG_TRACEPOINT_BENCHMARK=y
> >
> > # cd linux.git/tools/testing/selftests/ftrace/
> > # ./ftracetest test.d/ftrace/func_traceonoff_triggers.tc
> >
> > Locks up with a backtrace of:
> >
> > [ 614.186509] INFO: rcu_tasks detected stalls on tasks:
>
> Ah, but this is RCU-tasks! Which never sets rcu_dynticks.rcu_need_heavy_qs,
> thus needing a real context switch.
>
> Hey, when you said that synchronize_rcu_tasks() could take a very long
> time, I took you at your word! ;-)
>
> Does the following (untested, probably does not even build) patch make
> cond_resched() take a more peremptory approach to RCU-tasks?
And probably not. You are probably running CONFIG_PREEMPT=y (otherwise
RCU-tasks is trivial), so cond_resched() is a complete no-op:
static inline int _cond_resched(void) { return 0; }
I could make this call rcu_all_qs(), but I would not expect Peter Zijlstra
to be at all happy with that sort of change.
And the people who asked for the cond_resched() work probably aren't
going to be happy with the resumed proliferation of cond_resched_rcu_qs().
Hmmm... Grasping at straws... Could we make cond_resched() be something
like a tracepoint and instrument them with cond_resched_rcu_qs() if the
current RCU-tasks grace period ran for more that (say) a minute of its
ten-minute stall-warning span?
Thanx, Paul
next prev parent reply other threads:[~2018-02-25 18:17 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-01 19:21 [PATCH tip/core/rcu 0/10] Don not IPI offline CPUs, de-emphasize cond_resched_rcu_qs() Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 01/10] sched: Stop resched_cpu() from sending IPIs to offline CPUs Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 02/10] sched: Stop switched_to_rt() " Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 03/10] netfilter: Eliminate cond_resched_rcu_qs() in favor of cond_resched() Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 04/10] mm: " Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 05/10] workqueue: " Paul E. McKenney
2017-12-02 1:06 ` Lai Jiangshan
2017-12-04 18:28 ` Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 06/10] trace: " Paul E. McKenney
2018-02-24 20:12 ` Steven Rostedt
2018-02-25 17:49 ` Paul E. McKenney
2018-02-25 18:17 ` Paul E. McKenney [this message]
2018-02-25 18:39 ` Paul E. McKenney
2018-02-27 2:29 ` Steven Rostedt
2018-02-27 15:36 ` Paul E. McKenney
2018-02-28 23:12 ` Steven Rostedt
2018-03-01 1:21 ` Paul E. McKenney
2018-03-01 5:04 ` Steven Rostedt
2018-03-01 20:48 ` Paul E. McKenney
2018-03-02 20:06 ` Steven Rostedt
2018-03-03 0:54 ` Paul E. McKenney
2018-02-26 4:57 ` Steven Rostedt
2018-02-26 5:47 ` Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 07/10] softirq: " Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 08/10] fs: " Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 09/10] doc: " Paul E. McKenney
2017-12-01 19:21 ` [PATCH tip/core/rcu 10/10] rcu: Account for rcu_all_qs() in cond_resched() Paul E. McKenney
2017-12-02 8:56 ` Peter Zijlstra
2017-12-02 12:22 ` Paul E. McKenney
2017-12-02 13:55 ` Peter Zijlstra
2018-02-24 20:18 ` Steven Rostedt
2018-02-25 17:52 ` Paul E. McKenney
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