Re: slow sync rcu_tasks_trace

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Sep 10, 2020 at 11:33:58AM -0700, Alexei Starovoitov wrote:
> On 9/9/20 10:27 PM, Paul E. McKenney wrote:
> > On Wed, Sep 09, 2020 at 02:22:12PM -0700, Paul E. McKenney wrote:
> > > On Wed, Sep 09, 2020 at 02:04:47PM -0700, Paul E. McKenney wrote:
> > > > On Wed, Sep 09, 2020 at 12:48:28PM -0700, Alexei Starovoitov wrote:
> > > > > On Wed, Sep 09, 2020 at 12:39:00PM -0700, Paul E. McKenney wrote:
> > 
> > [ . . . ]
> > 
> > > > > > My plan is to try the following:
> > > > > > 
> > > > > > 1.	Parameterize the backoff sequence so that RCU Tasks Trace
> > > > > > 	uses faster rechecking than does RCU Tasks.  Experiment as
> > > > > > 	needed to arrive at a good backoff value.
> > > > > > 
> > > > > > 2.	If the tasks-list scan turns out to be a tighter bottleneck
> > > > > > 	than the backoff waits, look into parallelizing this scan.
> > > > > > 	(This seems unlikely, but the fact remains that RCU Tasks
> > > > > > 	Trace must do a bit more work per task than RCU Tasks.)
> > > > > > 
> > > > > > 3.	If these two approaches, still don't get the update-side
> > > > > > 	latency where it needs to be, improvise.
> > > > > > 
> > > > > > The exact path into mainline will of course depend on how far down this
> > > > > > list I must go, but first to get a solution.
> > > > > 
> > > > > I think there is a case of 4. Nothing is inside rcu_trace critical section.
> > > > > I would expect single ipi would confirm that.
> > > > 
> > > > Unless the task moves, yes.  So a single IPI should suffice in the
> > > > common case.
> > > 
> > > And what I am doing now is checking code paths.
> > 
> > And the following diff from a set of three patches gets my average
> > RCU Tasks Trace grace-period latencies down to about 20 milliseconds,
> > almost a 50x improvement from earlier today.
> > 
> > These are still quite rough and not yet suited for production use, but
> > I will be testing.  If that goes well, I hope to send a more polished
> > set of patches by end of day tomorrow, Pacific Time.  But if you get a
> > chance to test them, I would value any feedback that you might have.
> > 
> > These patches do not require hand-tuning, they instead adjust the
> > behavior according to CONFIG_TASKS_TRACE_RCU_READ_MB, which in turn
> > adjusts according to CONFIG_PREEMPT_RT.  So you should get the desired
> > latency reductions "out of the box", again, without tuning.
> 
> Great. Confirming improvement :)
> 
> time ./test_progs -t trampoline_count
> #101 trampoline_count:OK
> Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
> 
> real	0m2.897s
> user	0m0.128s
> sys	0m1.527s
> 
> This is without CONFIG_TASKS_TRACE_RCU_READ_MB, of course.

Good to hear, thank you!

or is more required?  I can tweak to get more.  There is never a free
lunch, though, and in this case the downside of further tweaking would
be greater CPU overhead.  Alternatively, I could just as easily tweak
it to be slower, thereby reducing the CPU overhead.

If I don't hear otherwise, I will assume that the current settings
work fine.

Of course, if people start removing thousands of BPF programs at one go,
I suspect that it will be necessary to provide a bulk-removal operation,
similar to some of the bulk-configuration-change operations provided by
networking.  The idea is to have a single RCU Tasks Trace grace period
cover all of the thousands of BPF removal operations.

							Thanx, Paul