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.
