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

Uladzislau Rezki
2023-03-15T17:12:35Z

On Tue, Mar 14, 2023 at 06:44:44PM -0400, Joel Fernandes wrote:
> On Mon, Mar 13, 2023 at 11:32 AM Uladzislau Rezki <urezki@gmail.com> wrote:
> >
> > On Mon, Mar 13, 2023 at 06:58:30AM -0700, Joel Fernandes wrote:
> > >
> > >
> > > > 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?
> > >
> > Do you mean before replacing the list(and after rcu_gp_cleanup()) a new
> > GP sequence can be initiated?
>
> It looks interesting, I am happy to try it on ChromeOS once you
> provide a patch, in case it improves something, even if that is
> suspend or boot time.
>
> I think the main concern I had was if you did not wait for a full
> grace period (which as you indicated, you would fix), you are not
> really measuring the long delays that the full grace period can cause
> so IMHO it is important to only measure once correctness is preserved
> by the modification. To that end, perhaps having rcutorture pass with
> your modification could be a vote of confidence before proceeding to
> performance tests.
>
No problem. Please note it is just a proof of concept. Here we go:

<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..a35b779471eb 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
@@ -1383,7 +1386,7 @@ static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
/*
* Initialize a new grace period. Return false if no grace period required.
*/
-static noinline_for_stack bool rcu_gp_init(void)
+static noinline_for_stack bool rcu_gp_init(struct llist_node **wait_list)
{
unsigned long flags;
unsigned long oldmask;
@@ -1409,6 +1412,12 @@ static noinline_for_stack bool rcu_gp_init(void)
return false;
}

+ /*
+ * Snapshot callers of synchronize_rcu() for which
+ * we guarantee a full grace period to be passed.
+ */
+ *wait_list = llist_del_all(&gp_wait_llist);
+
/* Advance to a new grace period and initialize state. */
record_gp_stall_check_time();
/* Record GP times before starting GP, hence rcu_seq_start(). */
@@ -1776,11 +1785,27 @@ 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;
+ int n = 0;
+
+ llist_for_each_safe(rcu, next, llist) {
+ complete(&((struct rcu_synchronize *) rcu)->completion);
+ n++;
+ }
+
+ if (n)
+ trace_printk("Awoken %d users.\n", n);
+}
+
/*
* Body of kthread that handles grace periods.
*/
static int __noreturn rcu_gp_kthread(void *unused)
{
+ struct llist_node *wait_list;
+
rcu_bind_gp_kthread();
for (;;) {

@@ -1795,7 +1820,7 @@ static int __noreturn rcu_gp_kthread(void *unused)
rcu_gp_torture_wait();
WRITE_ONCE(rcu_state.gp_state, RCU_GP_DONE_GPS);
/* Locking provides needed memory barrier. */
- if (rcu_gp_init())
+ if (rcu_gp_init(&wait_list))
break;
cond_resched_tasks_rcu_qs();
WRITE_ONCE(rcu_state.gp_activity, jiffies);
@@ -1811,6 +1836,9 @@ 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 synchronize_rcu() users. */
+ rcu_notify_gp_end(wait_list);
}
}

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 19bf6fa3ee6a..483997edd58e 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);
+ llist_add((struct llist_node *) &rs_array[i].head, &gp_wait_llist);
+
+ /* Kick a grace period if needed. */
+ (void) start_poll_synchronize_rcu();
}
}
<snip>

i do not think that it improves your boot time. My concern and what i
would like to fix is:

<snip>
<...>-29 [001] d..1. 21950.145313: rcu_batch_start: rcu_preempt CBs=3613 bl=28
...
<...>-29 [001] ..... 21950.152578: rcu_invoke_callback: rcu_preempt rhp=00000000b2d6dee8 func=__free_vm_area_struct.cfi_jt
<...>-29 [001] ..... 21950.152579: rcu_invoke_callback: rcu_preempt rhp=00000000a446f607 func=__free_vm_area_struct.cfi_jt
<...>-29 [001] ..... 21950.152580: rcu_invoke_callback: rcu_preempt rhp=00000000a5cab03b func=__free_vm_area_struct.cfi_jt
<...>-29 [001] ..... 21950.152581: rcu_invoke_callback: rcu_preempt rhp=0000000013b7e5ee func=__free_vm_area_struct.cfi_jt
<...>-29 [001] ..... 21950.152582: rcu_invoke_callback: rcu_preempt rhp=000000000a8ca6f9 func=__free_vm_area_struct.cfi_jt
<...>-29 [001] ..... 21950.152583: rcu_invoke_callback: rcu_preempt rhp=000000008f162ca8 func=wakeme_after_rcu.cfi_jt
<...>-29 [001] d..1. 21950.152625: rcu_batch_end: rcu_preempt CBs-invoked=3612 idle=....
<snip>

i grabbed that good example(our phone device) where a user of synchronize_rcu() is "un-blocked"
as last since its callback was the last in a list.

--
Uladzislau Rezki