Messages in this thread | | | From | Eric Dumazet <> | Date | Fri, 1 Apr 2022 06:24:13 -0700 | Subject | Re: [BUG] rcu-tasks : should take care of sparse cpu masks |
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On Fri, Apr 1, 2022 at 6:01 AM Paul E. McKenney <paulmck@kernel.org> wrote: > > On Thu, Mar 31, 2022 at 09:39:02PM -0700, Eric Dumazet wrote: > > On Thu, Mar 31, 2022 at 5:06 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > > > > > On Thu, Mar 31, 2022 at 04:28:04PM -0700, Eric Dumazet wrote: > > > > On Thu, Mar 31, 2022 at 4:13 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > > > > > > > > > The initial setting of ->percpu_enqueue_shift forces all in-range CPU > > > > > IDs to shift down to zero. The grace-period kthread is allowed to run > > > > > where it likes. The callback lists are protected by locking, even in > > > > > the case of local access, so this should be safe. > > > > > > > > > > Or am I missing your point? > > > > > > > > > > > > > In fact I have been looking at this code, because we bisected a > > > > regression back to this patch: > > > > > > > > 4fe192dfbe5ba9780df699d411aa4f25ba24cf61 rcu-tasks: Shorten > > > > per-grace-period sleep for RCU Tasks Trace > > > > > > > > It is very possible the regression comes because the RCU task thread > > > > is using more cpu cycles, from 'CPU 0' where our system daemons are > > > > pinned. > > > > > > Heh! I did express that concern when creating that patch, but was > > > assured that the latency was much more important. > > > > > > Yes, that patch most definitely increases CPU utilization during RCU Tasks > > > Trace grace periods. If you can tolerate longer grace-period latencies, > > > it might be worth toning it down a bit. The ask was for about twice > > > the latency I achieved in my initial attempt, and I made the mistake of > > > forwarding that attempt out for testing. They liked the shorter latency > > > very much, and objected strenuously to the thought that I might detune > > > it back to the latency that they originally asked for. ;-) > > > > > > But I can easily provide the means to detune it through use of a kernel > > > boot parameter or some such, if that would help. > > > > > > > But I could not spot where the RCU task kthread is forced to run on CPU 0. > > > > > > I never did intend this kthread be bound anywhere. RCU's policy is > > > that any binding of its kthreads is the responsibility of the sysadm, > > > be that carbon-based or otherwise. > > > > > > But this kthread is spawned early enough that only CPU 0 is online, > > > so maybe the question is not "what is binding it to CPU 0?" but rather > > > "why isn't something kicking it off of CPU 0?" > > > > I guess the answer to this question can be found in the following > > piece of code :) > > > > rcu_read_lock(); > > for_each_process_thread(g, t) > > rtp->pertask_func(t, &holdouts); > > rcu_read_unlock(); > > > > > > With ~150,000 threads on a 256 cpu host, this holds current cpu for > > very long times: > > > > rcu_tasks_trace 11 [017] 5010.544762: > > probe:rcu_tasks_wait_gp: (ffffffff963fb4b0) > > rcu_tasks_trace 11 [017] 5010.600396: > > probe:rcu_tasks_trace_postscan: (ffffffff963fb7c0) > > So about 55 milliseconds for the tasklist scan, correct? Or am I > losing the plot here? > > > rcu_tasks_trace 11 [022] 5010.618783: > > probe:check_all_holdout_tasks_trace: (ffffffff963fb850) > > rcu_tasks_trace 11 [022] 5010.618840: > > probe:rcu_tasks_trace_postgp: (ffffffff963fba70) > > > > In this case, CPU 22 is the victim, not CPU 0 :) > > My faith in the scheduler is restored! ;-) > > My position has been that this tasklist scan does not need to be broken > up because it should happen only when a sleepable BPF program is removed, > which is a rare event.
Hmm... what about bpf_sk_storage_free() ?
Definitely not a rare event.
> > In addition, breaking up this scan is not trivial, because as far as I > know there is no way to force a given task to stay in the list. I would > have to instead use something like rcu_lock_break(), and restart the > scan if either of the nailed-down pair of tasks was removed from the list. > In a system where tasks were coming and going very frequently, it might > be that such a broken-up scan would never complete. > > I can imagine tricks where the nailed-down tasks are kept on a list, > and the nailed-downness is moved to the next task when those tasks > are removed. I can also imagine a less-than-happy response to such > a proposal. > > So I am not currently thinking in terms of breaking up this scan. > > Or is there some trick that I am missing? > > In the meantime, a simple patch that reduces the frequency of the scan > by a factor of two. But this would not be the scan of the full tasklist, > but rather the frequency of the calls to check_all_holdout_tasks_trace(). > And the total of these looks to be less than 20 milliseconds, if I am > correctly interpreting your trace. And most of that 20 milliseconds > is sleeping. > > Nevertheless, the patch is at the end of this email. > > Other than that, I could imagine batching removal of sleepable BPF > programs and using a single grace period for all of their trampolines. > But are there enough sleepable BPF programs ever installed to make this > a useful approach? > > Or is the status quo in fact acceptable? (Hey, I can dream, can't I?) > > Thanx, Paul > > > > > I attempted to backport to our kernel all related patches that were > > > > not yet backported, > > > > and we still see a regression in our tests. > > > > > > The per-grace-period CPU consumption of rcu_tasks_trace was intentionally > > > increased by the above commit, and I never have done anything to reduce > > > that CPU consumption. In part because you are the first to call my > > > attention to it. > > > > > > Oh, and one other issue that I very recently fixed, that has not > > > yet reached mainline, just in case it matters. If you are building a > > > CONFIG_PREEMPT_NONE=y or CONFIG_PREEMPT_VOLUNTARY=y kernel, but also have > > > CONFIG_RCU_TORTURE_TEST=m (or, for that matter, =y, but please don't in > > > production!), then your kernel will use RCU Tasks instead of vanilla RCU. > > > (Note well, RCU Tasks, not RCU Tasks Trace, the latter being necessaary > > > for sleepable BPF programs regardless of kernel .config). > > > > > > > Please ignore the sha1 in this current patch series, this is only to > > > > show my current attempt to fix the regression in our tree. > > > > > > > > 450b3244f29b rcu-tasks: Don't remove tasks with pending IPIs from holdout list > > > > 5f88f7e9cc36 rcu-tasks: Create per-CPU callback lists > > > > 1a943d0041dc rcu-tasks: Introduce ->percpu_enqueue_shift for dynamic > > > > queue selection > > > > ea5289f12fce rcu-tasks: Convert grace-period counter to grace-period > > > > sequence number > > > > 22efd5093c3b rcu/segcblist: Prevent useless GP start if no CBs to accelerate > > > > 16dee1b3babf rcu: Implement rcu_segcblist_is_offloaded() config dependent > > > > 8cafaadb6144 rcu: Add callbacks-invoked counters > > > > 323234685765 rcu/tree: Make rcu_do_batch count how many callbacks were executed > > > > f48f3386a1cc rcu/segcblist: Add additional comments to explain smp_mb() > > > > 4408105116de rcu/segcblist: Add counters to segcblist datastructure > > > > 4a0b89a918d6 rcu/tree: segcblist: Remove redundant smp_mb()s > > > > 38c0d18e8740 rcu: Add READ_ONCE() to rcu_do_batch() access to rcu_divisor > > > > 0b5d1031b509 rcu/segcblist: Add debug checks for segment lengths > > > > 8a82886fbf02 rcu_tasks: Convert bespoke callback list to rcu_segcblist structure > > > > cbd452a5c01f rcu-tasks: Use spin_lock_rcu_node() and friends > > > > 073222be51f3 rcu-tasks: Add a ->percpu_enqueue_lim to the rcu_tasks structure > > > > 5af10fb0f8fb rcu-tasks: Abstract checking of callback lists > > > > d3e8be598546 rcu-tasks: Abstract invocations of callbacks > > > > 65784460a392 rcu-tasks: Use workqueues for multiple > > > > rcu_tasks_invoke_cbs() invocations > > > > dd6413e355f1 rcu-tasks: Make rcu_barrier_tasks*() handle multiple > > > > callback queues > > > > 2499cb3c438e rcu-tasks: Add rcupdate.rcu_task_enqueue_lim to set > > > > initial queueing > > > > a859f409a503 rcu-tasks: Count trylocks to estimate call_rcu_tasks() contention > > > > 4ab253ca056e rcu-tasks: Avoid raw-spinlocked wakeups from > > > > call_rcu_tasks_generic() > > > > e9a3563fe76e rcu-tasks: Use more callback queues if contention encountered > > > > 4023187fe31d rcu-tasks: Use separate ->percpu_dequeue_lim for callback > > > > dequeueing > > > > 533be3bd47c3 rcu: Provide polling interfaces for Tree RCU grace periods > > > > f7e5a81d7953 rcu-tasks: Use fewer callbacks queues if callback flood ends > > > > bb7ad9078e1b rcu-tasks: Fix computation of CPU-to-list shift counts > > > > d9cebde55539 rcu-tasks: Use order_base_2() instead of ilog2() > > > > 95606f1248f5 rcu-tasks: Set ->percpu_enqueue_shift to zero upon contention > > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h > index 65d6e21a607a..141e2b4c70cc 100644 > --- a/kernel/rcu/tasks.h > +++ b/kernel/rcu/tasks.h > @@ -1640,10 +1640,10 @@ static int __init rcu_spawn_tasks_trace_kthread(void) > rcu_tasks_trace.gp_sleep = HZ / 10; > rcu_tasks_trace.init_fract = HZ / 10; > } else { > - rcu_tasks_trace.gp_sleep = HZ / 200; > + rcu_tasks_trace.gp_sleep = HZ / 100; > if (rcu_tasks_trace.gp_sleep <= 0) > rcu_tasks_trace.gp_sleep = 1; > - rcu_tasks_trace.init_fract = HZ / 200; > + rcu_tasks_trace.init_fract = HZ / 100; > if (rcu_tasks_trace.init_fract <= 0) > rcu_tasks_trace.init_fract = 1; > }
It seems that if the scan time is > 50ms in some common cases (at least at Google scale), the claim of having a latency of 10ms is not reasonable.
Given that, I do not think bpf_sk_storage_free() can/should use call_rcu_tasks_trace(), we probably will have to fix this soon (or revert from our kernels)
Thanks.
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