Messages in this thread | | | Date | Sun, 25 Feb 2018 10:17:30 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 06/10] trace: Eliminate cond_resched_rcu_qs() in favor of cond_resched() |
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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
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