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" <paul...@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? Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 0c337f5ba3c4..5155fe5e7702 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1088,12 +1088,16 @@ EXPORT_SYMBOL_GPL(rcu_is_watching); void rcu_request_urgent_qs_task(struct task_struct *t) { int cpu; + struct rcu_dynticks *rdtp; barrier(); cpu = task_cpu(t); if (!task_curr(t)) return; /* This task is not running on that CPU. */ - smp_store_release(per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, cpu), true); + rdtp = per_cpu_ptr(&rcu_dynticks, cpu); + WRITE_ONCE(rdtp->rcu_need_heavy_qs, true); + /* Store rcu_need_heavy_qs before rcu_urgent_qs. */ + smp_store_release(&rdtp->rcu_urgent_qs, true); } #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)