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

Uladzislau Rezki
2023-03-13T09:51:49Z

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);
+}
+
/*
* 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