Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node`

On Thu, Dec 01, 2016 at 05:36:14PM +0100, Peter Zijlstra wrote:

On Thu, Dec 01, 2016 at 04:40:24AM -0800, Paul E. McKenney wrote:

quoted

On Thu, Dec 01, 2016 at 06:30:35AM +0100, Peter Zijlstra wrote:

quoted

quoted

Sure, we all dislike IPIs, but I'm thinking this half-way point is
sensible, no point in issuing user visible annoyance if indeed we can
prod things back to life, no?

Only if we utterly fail to make it respond should we bug the user with
our failure..

Sold!  ;-)

I will put together a patch later today.

My intent is to hold off on the "upgrade cond_resched()" patch, one
step at a time.  Longer term, I do very much like the idea of having
cond_resched() do both scheduling and RCU quiescent states, assuming
that this avoids performance pitfalls.

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?

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?

#define cond_resched_rcu_qs() \
do { \
	if (!should_resched(current) || !cond_resched()) \
		rcu_note_voluntary_context_switch(current); \
} while (0)

							Thanx, Paul

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