On Wed, Nov 01, 2023 at 09:35:30PM -0700, Paul E. McKenney wrote:
> On Mon, Oct 30, 2023 at 02:12:52PM +0100, Uladzislau Rezki (Sony) wrote:
> > A call to a synchronize_rcu() can be optimized from a latency
> > point of view. Workloads which depend on this can benefit of it.
> >
> > The delay of wakeme_after_rcu() callback, which unblocks a waiter,
> > depends on several factors:
> >
> > - how fast a process of offloading is started. Combination of:
> > - !CONFIG_RCU_NOCB_CPU/CONFIG_RCU_NOCB_CPU;
> > - !CONFIG_RCU_LAZY/CONFIG_RCU_LAZY;
> > - other.
> > - when started, invoking path is interrupted due to:
> > - time limit;
> > - need_resched();
> > - if limit is reached.
> > - where in a nocb list it is located;
> > - how fast previous callbacks completed;
> >
> > Example:
> >
> > 1. On our embedded devices i can easily trigger the scenario when
> > it is a last in the list out of ~3600 callbacks:
> >
> > <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>
> >
> > 2. We use cpuset/cgroup to classify tasks and assign them into
> > different cgroups. For example "backgrond" group which binds tasks
> > only to little CPUs or "foreground" which makes use of all CPUs.
> > Tasks can be migrated between groups by a request if an acceleration
> > is needed.
> >
> > See below an example how "surfaceflinger" task gets migrated.
> > Initially it is located in the "system-background" cgroup which
> > allows to run only on little cores. In order to speed it up it
> > can be temporary moved into "foreground" cgroup which allows
> > to use big/all CPUs:
> >
> > cgroup_attach_task():
> > -> cgroup_migrate_execute()
> > -> cpuset_can_attach()
> > -> percpu_down_write()
> > -> rcu_sync_enter()
> > -> synchronize_rcu()
> > -> now move tasks to the new cgroup.
> > -> cgroup_migrate_finish()
> >
> > <snip>
> > rcuop/1-29 [000] ..... 7030.528570: rcu_invoke_callback: rcu_preempt rhp=00000000461605e0 func=wakeme_after_rcu.cfi_jt
> > PERFD-SERVER-1855 [000] d..1. 7030.530293: cgroup_attach_task: dst_root=3 dst_id=22 dst_level=1 dst_path=/foreground pid=1900 comm=surfaceflinger
> > TimerDispatch-2768 [002] d..5. 7030.537542: sched_migrate_task: comm=surfaceflinger pid=1900 prio=98 orig_cpu=0 dest_cpu=4
> > <snip>
> >
> > "Boosting a task" depends on synchronize_rcu() latency:
> >
> > - first trace shows a completion of synchronize_rcu();
> > - second shows attaching a task to a new group;
> > - last shows a final step when migration occurs.
> >
> > 3. To address this drawback, maintain a separate track that consists
> > of synchronize_rcu() callers only. After completion of a grace period
> > users are awaken directly, it is limited by allowed threshold, others
> > are deferred(if still exist) to a worker to complete the rest.
> >
> > 4. This patch reduces the latency of synchronize_rcu() approximately
> > by ~30-40% on synthetic tests. The real test case, camera launch time,
> > shows(time is in milliseconds):
> >
> > 1-run 542 vs 489 improvement 9%
> > 2-run 540 vs 466 improvement 13%
> > 3-run 518 vs 468 improvement 9%
> > 4-run 531 vs 457 improvement 13%
> > 5-run 548 vs 475 improvement 13%
> > 6-run 509 vs 484 improvement 4%
> >
> > Synthetic test:
> >
> > Hardware: x86_64 64 CPUs, 64GB of memory
> >
> > - 60K tasks(simultaneous);
> > - each task does(1000 loops)
> > synchronize_rcu();
> > kfree(p);
> >
> > default: CONFIG_RCU_NOCB_CPU: takes 323 seconds to complete all users;
> > patch: CONFIG_RCU_NOCB_CPU: takes 240 seconds to complete all users.
> >
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
>
> This looks pretty close! Some questions and comments below, much of
> which being what I managed not to write down in earlier discussions. :-/
>
Sounds good :)
> > ---
> > kernel/rcu/tree.c | 154 +++++++++++++++++++++++++++++++++++++++++-
> > kernel/rcu/tree_exp.h | 2 +-
> > 2 files changed, 154 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 78554e7181dd..f04846b543de 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1384,6 +1384,125 @@ static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
> > raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > }
> >
> > +/*
> > + * There are three lists for handling synchronize_rcu() users.
> > + * A first list corresponds to new coming users, second for users
> > + * which wait for a grace period and third is for which a grace
> > + * period is passed.
> > + */
> > +static struct sr_normal_state {
> > + struct llist_head srs_next; /* request a GP users. */
> > + struct llist_head srs_wait; /* wait for GP users. */
> > + struct llist_head srs_done; /* ready for GP users. */
> > +
> > + /*
> > + * In order to add a batch of nodes to already
> > + * existing srs-done-list, a tail of srs-wait-list
> > + * is maintained.
> > + */
> > + struct llist_node *srs_wait_tail;
> > +} sr;
>
> It would be good to put these fields into the rcu_state structure.
> Unlike kfree_rcu(), I have no ambitions for the mm guys ever taking
> this one. ;-)
>
OK. I will rework it. It is better to keep it in one solid place.
> > +/* Disabled by default. */
> > +static int rcu_normal_wake_from_gp;
> > +module_param(rcu_normal_wake_from_gp, int, 0644);
> > +
> > +static void rcu_sr_normal_complete(struct llist_node *node)
> > +{
> > + struct rcu_synchronize *rs = container_of(
> > + (struct rcu_head *) node, struct rcu_synchronize, head);
> > + unsigned long oldstate = (unsigned long) rs->head.func;
> > +
> > + WARN_ONCE(!poll_state_synchronize_rcu(oldstate),
> > + "A full grace period is not passed yet: %lu",
> > + rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
>
> This needs to either:
>
> 1. Use poll_state_synchronize_rcu_full(), or
>
> 2. Avoid firing unless expedited grace periods have been disabled.
> Note that forcing synchronize_rcu() to synchronize_rcu_expedited()
> does not help because there might still be call_rcu() invocations
> advancing normal grace periods.
>
> As it stands, you can have false-positive WARN_ONCE()s. These can happen
> when a normal and an expedited grace period overlap in time.
>
I prefer an option [2]:
<snip>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 189975f57e78..85f3e7d3642e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1413,7 +1413,7 @@ static void rcu_sr_normal_complete(struct llist_node *node)
(struct rcu_head *) node, struct rcu_synchronize, head);
unsigned long oldstate = (unsigned long) rs->head.func;
- WARN_ONCE(!poll_state_synchronize_rcu(oldstate),
+ WARN_ONCE(!rcu_gp_is_expedited() && !poll_state_synchronize_rcu(oldstate),
"A full grace period is not passed yet: %lu",
rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
<snip>
> > + /* Finally. */
> > + complete(&rs->completion);
> > +}
> > +
> > +static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > +{
> > + struct llist_node *done, *rcu, *next;
> > +
> > + done = llist_del_all(&sr.srs_done);
> > + if (!done)
> > + return;
> > +
> > + llist_for_each_safe(rcu, next, done)
> > + rcu_sr_normal_complete(rcu);
> > +}
> > +static DECLARE_WORK(sr_normal_gp_cleanup, rcu_sr_normal_gp_cleanup_work);
> > +
> > +/*
> > + * This is hard-coded and it is a maximum number of
> > + * synchronize_rcu() users(might be +1 extra), which
> > + * are awaken directly by the rcu_gp_kthread(). The
> > + * reset is deferred to a dedicated worker.
>
> s/reset/rest/
>
Typo. Thanks!
> > + */
> > +#define MAX_SR_WAKE_FROM_GP 5
> > +
> > +/*
> > + * Helper function for rcu_gp_cleanup().
> > + */
> > +static void rcu_sr_normal_gp_cleanup(void)
> > +{
> > + struct llist_node *head, *tail, *pos;
> > + int i = 0;
> > +
> > + if (llist_empty(&sr.srs_wait))
> > + return;
> > +
> > + tail = READ_ONCE(sr.srs_wait_tail);
> > + head = __llist_del_all(&sr.srs_wait);
> > +
> > + llist_for_each_safe(pos, head, head) {
> > + rcu_sr_normal_complete(pos);
> > +
> > + if (++i == MAX_SR_WAKE_FROM_GP) {
> > + /* If last, process it also. */
> > + if (head && !head->next)
> > + continue;
> > + break;
>
> Save a line this way?
>
> if (!head || head->next)
> break;
I would like to process clients from a GP-kthread but i am not
allowed to offload all by the threshold. If last client is left
i process it also, since we lose nothing and instead of kicking
a worker to do a final job we process it right away.
>
> > + }
> > + }
> > +
> > + if (head) {
> > + /* Can be not empty. */
> > + llist_add_batch(head, tail, &sr.srs_done);
> > + queue_work(system_highpri_wq, &sr_normal_gp_cleanup);
> > + }
> > +}
> > +
> > +/*
> > + * Helper function for rcu_gp_init().
> > + */
> > +static void rcu_sr_normal_gp_init(void)
> > +{
> > + struct llist_node *head, *tail;
> > +
> > + if (llist_empty(&sr.srs_next))
> > + return;
> > +
> > + tail = llist_del_all(&sr.srs_next);
> > + head = llist_reverse_order(tail);
>
> Hmmm... I am not loving this list-reverse operation. Once someone
> figures out how to generate a long list, it is going to hurt quite badly.
>
> Except... Why do we need to reverse the list in the first place?
> It appears that one reason is to be able to get the tail of the list.
> Is it also necessary to do the wakeups in order, or could they be
> reversed? It seems like they should -- the average latency would remain
> the same. If so, couldn't we have a single llist with two pointers into
> it (more accurately, to its tail pointers), one for the first done item,
> and the other for the first item waiting on the current grace period?
>
> Then it would not be necessary to reverse the list, nor would it be
> necessary to move elemetns from one list to another. Just copy one
> pointer to the next.
>
> If it ever becomes necessary to put extra elements back, which would be
> challenging if there were no other elements in the list. The usual way
> to handle this is to have a dummy element to isolate the enqueuers from
> the requeuer. The GP kthread then enqueues the dummy element if the
> list is empty, which means that enqueue and optimized wakeup are never
> looking at the same pointer. Alternatively, just use dummy elements to
> mark the segments in the list, with the added pointers always referencing
> these dummy elements. Might need a VC to make this make sense...
>
> Or is there some reason that this approach would break things?
>
Hm.. I need to rework it i agree. Reversing the list is a good thing
if we would like to reduce the worst case, i mean latency. Because we
kick users which waited the most. But it is not critical, it is just
a micro optimization and if we have it - fine, if not - no problem.
Can we proceed as it is now? I am asking, because i do not find it too
critical. My tests show only 1% difference doing 60K syncing. I need
some time to rework it more carefully.
I was thinking about read_lock()/write_lock() since we have many readers
and only one writer. But i do not really like it either.
> > + /*
> > + * A waiting list of GP should be empty on this step,
> > + * since a GP-kthread, rcu_gp_init() -> gp_cleanup(),
> > + * rolls it over. If not, it is a BUG, warn a user.
> > + */
> > + WARN_ON_ONCE(!llist_empty(&sr.srs_wait));
> > +
> > + WRITE_ONCE(sr.srs_wait_tail, tail);
> > + __llist_add_batch(head, tail, &sr.srs_wait);
>
> So sr.srs_wait_tail keeps a pointer into the list, and acts kind of like
> a rcu_segcblist tail pointer.
>
Let me check!
> > +}
> > +
> > +static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> > +{
> > + llist_add((struct llist_node *) &rs->head, &sr.srs_next);
>
> s/&rs->head/&rs->head.next/?
>
Same, let me check it.
> > +}
> > +
> > /*
> > * Initialize a new grace period. Return false if no grace period required.
> > */
> > @@ -1418,6 +1537,7 @@ static noinline_for_stack bool rcu_gp_init(void)
> > /* Record GP times before starting GP, hence rcu_seq_start(). */
> > rcu_seq_start(&rcu_state.gp_seq);
> > ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
> > + rcu_sr_normal_gp_init();
> > trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
> > rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
> > raw_spin_unlock_irq_rcu_node(rnp);
> > @@ -1787,6 +1907,9 @@ static noinline void rcu_gp_cleanup(void)
> > }
> > raw_spin_unlock_irq_rcu_node(rnp);
> >
> > + // Make synchronize_rcu() users aware of the end of old grace period.
> > + rcu_sr_normal_gp_cleanup();
> > +
> > // If strict, make all CPUs aware of the end of the old grace period.
> > if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
> > on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
> > @@ -3500,6 +3623,35 @@ static int rcu_blocking_is_gp(void)
> > return true;
> > }
> >
> > +/*
> > + * Helper function for the synchronize_rcu() API.
> > + */
> > +static void synchronize_rcu_normal(void)
> > +{
> > + struct rcu_synchronize rs;
> > +
> > + if (READ_ONCE(rcu_normal_wake_from_gp)) {
> > + init_rcu_head_on_stack(&rs.head);
> > + init_completion(&rs.completion);
> > +
> > + /*
> > + * This code might be preempted, therefore take a GP
> > + * snapshot before adding a request.
> > + */
> > + rs.head.func = (void *) get_state_synchronize_rcu();
> > + rcu_sr_normal_add_req(&rs);
> > +
> > + /* Kick a GP and start waiting. */
> > + (void) start_poll_synchronize_rcu();
> > +
> > + /* Now we can wait. */
> > + wait_for_completion(&rs.completion);
> > + destroy_rcu_head_on_stack(&rs.head);
> > + } else {
> > + wait_rcu_gp(call_rcu_hurry);
> > + }
>
> Please save some indentation as follows:
>
> if (!READ_ONCE(rcu_normal_wake_from_gp)) {
> wait_rcu_gp(call_rcu_hurry);
> return;
> }
> init_rcu_head_on_stack(&rs.head);
> ...
>
> Same number of lines.
>
OK. I will do that!
Appreciate for review :)
--
Uladzislau Rezki
