Re: [PATCH tip/core/rcu 06/10] trace: Eliminate cond_resched_rcu_qs() in favor of cond_resched()

["Paul E. McKenney" <paulmck@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