Re: [PATCH v2 1/3] rcu: Reduce synchronize_rcu() latency

From: Paul E. McKenney
Date: Tue Nov 07 2023 - 00:32:12 EST

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On Mon, Nov 06, 2023 at 01:04:42PM +0100, Uladzislau Rezki wrote:
> 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.

Very good, thank you!

> > > +/* 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>

In this case, don't you instead need rcu_gp_is_normal()?

Ah, but this thing can be changed via sysfs. For the diagnostic
to be reliable, expedited grace periods have to have been disabled
for the full time from the start_poll_synchronize_rcu() to the final
poll_state_synchronize_rcu(). And userspace can toggle rcu_normal via
sysfs as often and as many times as they like. :-/

I can imagine ways around this, but they are a bit ugly. They end
up being things like recording a timestamp on every sysfs change to
rcu_normal, and then using that timestamp to deduce whether there could
possibly have been sysfs activity on rcu_normal in the meantime.

It feels like it should be so easy... ;-)

> > > + /* 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.

Unless I blew my de Morgan transformation (which I might well have done),
the one-line approach should be functionally identical to your original.

> > > + }
> > > + }
> > > +
> > > + 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 am concerned about latencies. These sorts of things can bit us
pretty hard.

> 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.

This might be a hint that we should have multiple lists, perhaps one
per CPU. Or lock contention could be used to trigger the transition
from a single list to multiple lists. as is done in SRCU and tasks RCU.

But I bet that there are several ways to make things work.

> > > + /*
> > > + * 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 :)

I will try to be faster next time!

Thanx, Paul

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