Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

Joel Fernandes
2023-03-13T13:59:22Z

> On Mar 13, 2023, at 2:51 AM, Uladzislau Rezki <urezki@gmail.com> wrote:
>
> ﻿On Fri, Mar 10, 2023 at 10:24:34PM -0800, Paul E. McKenney wrote:
>>> On Fri, Mar 10, 2023 at 09:55:02AM +0100, Uladzislau Rezki wrote:
>>> On Thu, Mar 09, 2023 at 10:10:56PM +0000, Joel Fernandes wrote:
>>>> On Thu, Mar 09, 2023 at 01:57:42PM +0100, Uladzislau Rezki wrote:
>>>> [..]
>>>>>>>>>> See this commit:
>>>>>>>>>>
>>>>>>>>>> 3705b88db0d7cc ("rcu: Add a module parameter to force use of
>>>>>>>>>> expedited RCU primitives")
>>>>>>>>>>
>>>>>>>>>> Antti provided this commit precisely in order to allow Android
>>>>>>>>>> devices to expedite the boot process and to shut off the
>>>>>>>>>> expediting at a time of Android userspace's choosing. So Android
>>>>>>>>>> has been making this work for about ten years, which strikes me
>>>>>>>>>> as an adequate proof of concept. ;-)
>>>>>>>>>
>>>>>>>>> Thanks for the pointer. That's true. Looking at Android sources, I
>>>>>>>>> find that Android Mediatek devices at least are setting
>>>>>>>>> rcu_expedited to 1 at late stage of their userspace boot (which is
>>>>>>>>> weird, it should be set to 1 as early as possible), and
>>>>>>>>> interestingly I cannot find them resetting it back to 0!. Maybe
>>>>>>>>> they set rcu_normal to 1? But I cannot find that either. Vlad? :P
>>>>>>>>
>>>>>>>> Interesting. Though this is consistent with Antti's commit log,
>>>>>>>> where he talks about expediting grace periods but not unexpediting
>>>>>>>> them.
>>>>>>>>
>>>>>>> Do you think we need to unexpedite it? :))))
>>>>>>
>>>>>> Android runs on smallish systems, so quite possibly not!
>>>>>>
>>>>> We keep it enabled and never unexpedite it. The reason is a performance. I
>>>>> have done some app-launch time analysis with enabling and disabling of it.
>>>>>
>>>>> An expedited case is much better when it comes to app launch time. It
>>>>> requires ~25% less time to run an app comparing with unexpedited variant.
>>>>> So we have a big gain here.
>>>>
>>>> Wow, that's huge. I wonder if you can dig deeper and find out why that is so
>>>> as the callbacks may need to be synchronize_rcu_expedited() then, as it could
>>>> be slowing down other usecases! I find it hard to believe, real-time
>>>> workloads will run better without those callbacks being always-expedited if
>>>> it actually gives back 25% in performance!
>>>>
>>> I can dig further, but on a high level i think there are some spots
>>> which show better performance if expedited is set. I mean synchronize_rcu()
>>> becomes as "less blocking a context" from a time point of view.
>>>
>>> The problem of a regular synchronize_rcu() is - it can trigger a big latency
>>> delays for a caller. For example for nocb case we do not know where in a list
>>> our callback is located and when it is invoked to unblock a caller.
>>
>> True, expedited RCU grace periods do not have this callback-invocation
>> delay that normal RCU does.
>>
>>> I have already mentioned somewhere. Probably it makes sense to directly wake-up
>>> callers from the GP kthread instead and not via nocb-kthread that invokes our callbacks
>>> one by one.
>>
>> Makes sense, but it is necessary to be careful. Wakeups are not fast,
>> so making the RCU grace-period kthread do them all sequentially is not
>> a strategy to win. For example, note that the next expedited grace
>> period can start before the previous expedited grace period has finished
>> its wakeups.
>>
> I hove done a small and quick prototype:
>
> <snip>
> diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
> index 699b938358bf..e1a4cca9a208 100644
> --- a/include/linux/rcupdate_wait.h
> +++ b/include/linux/rcupdate_wait.h
> @@ -9,6 +9,8 @@
> #include <linux/rcupdate.h>
> #include <linux/completion.h>
>
> +extern struct llist_head gp_wait_llist;
> +
> /*
> * Structure allowing asynchronous waiting on RCU.
> */
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index ee27a03d7576..50b81ca54104 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -113,6 +113,9 @@ int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
> int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
> int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
>
> +/* Waiters for a GP kthread. */
> +LLIST_HEAD(gp_wait_llist);
> +
> /*
> * The rcu_scheduler_active variable is initialized to the value
> * RCU_SCHEDULER_INACTIVE and transitions RCU_SCHEDULER_INIT just before the
> @@ -1776,6 +1779,14 @@ static noinline void rcu_gp_cleanup(void)
> on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
> }
>
> +static void rcu_notify_gp_end(struct llist_node *llist)
> +{
> + struct llist_node *rcu, *next;
> +
> + llist_for_each_safe(rcu, next, llist)
> + complete(&((struct rcu_synchronize *) rcu)->completion);

This looks broken to me, so the synchronize will complete even
if it was called in the middle of an ongoing GP?

Thanks,

- Joel

> +}
> +
> /*
> * Body of kthread that handles grace periods.
> */
> @@ -1811,6 +1822,9 @@ static int __noreturn rcu_gp_kthread(void *unused)
> WRITE_ONCE(rcu_state.gp_state, RCU_GP_CLEANUP);
> rcu_gp_cleanup();
> WRITE_ONCE(rcu_state.gp_state, RCU_GP_CLEANED);
> +
> + /* Wake-app all users. */
> + rcu_notify_gp_end(llist_del_all(&gp_wait_llist));
> }
> }
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 19bf6fa3ee6a..1de7c328a3e5 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -426,7 +426,10 @@ void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
> if (j == i) {
> init_rcu_head_on_stack(&rs_array[i].head);
> init_completion(&rs_array[i].completion);
> - (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu);
> +
> + /* Kick a grace period if needed. */
> + (void) start_poll_synchronize_rcu();
> + llist_add((struct llist_node *) &rs_array[i].head, &gp_wait_llist);
> }
> }
> <snip>
>
> and did some experiments in terms of performance and comparison. A test case is:
>
> thread_X:
> synchronize_rcu();
> kfree(ptr);
>
> below are results with running 10 parallel workers running 1000 times of mentioned
> test scenario:
>
> # default(NOCB)
> [ 29.322944] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 17286604 usec
> [ 29.325759] All test took worker0=63964052068 cycles
> [ 29.327255] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 23414575 usec
> [ 29.329974] All test took worker1=86638822563 cycles
> [ 29.331460] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 23357988 usec
> [ 29.334205] All test took worker2=86429439193 cycles
> [ 29.350808] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 17174001 usec
> [ 29.353553] All test took worker3=63547397954 cycles
> [ 29.355039] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 17141904 usec
> [ 29.357770] All test took worker4=63428630877 cycles
> [ 29.374831] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 23397952 usec
> [ 29.377577] All test took worker5=86577316353 cycles
> [ 29.398809] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 17142038 usec
> [ 29.401549] All test took worker6=63429124938 cycles
> [ 29.414828] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 17158248 usec
> [ 29.417574] All test took worker7=63489107118 cycles
> [ 29.438811] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 18102109 usec
> [ 29.441550] All test took worker8=66981588881 cycles
> [ 29.462826] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 23446042 usec
> [ 29.465561] All test took worker9=86755258455 cycles
>
> # patch(NOCB)
> [ 14.720986] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837883 usec
> [ 14.723753] All test took worker0=32702015768 cycles
> [ 14.740386] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837750 usec
> [ 14.743076] All test took worker1=32701525814 cycles
> [ 14.760350] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837734 usec
> [ 14.763036] All test took worker2=32701466281 cycles
> [ 14.780369] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837707 usec
> [ 14.783057] All test took worker3=32701364901 cycles
> [ 14.800352] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837730 usec
> [ 14.803041] All test took worker4=32701449927 cycles
> [ 14.820355] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837724 usec
> [ 14.823048] All test took worker5=32701428134 cycles
> [ 14.840359] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837705 usec
> [ 14.843052] All test took worker6=32701356465 cycles
> [ 14.860322] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837742 usec
> [ 14.863005] All test took worker7=32701494475 cycles
> [ 14.880363] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837750 usec
> [ 14.883081] All test took worker8=32701525074 cycles
> [ 14.900362] Summary: kvfree_rcu_1_arg_vmalloc_test loops: 1000 avg: 8837918 usec
> [ 14.903065] All test took worker9=32702145379 cycles
>
> --
> Uladzislau Rezki