Message-ID: <70578164-6c12-47ca-9528-163b688c1b47@paulmck-laptop>
Date:   Wed, 1 Nov 2023 21:35:30 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     RCU <rcu@...r.kernel.org>,
        Neeraj upadhyay <Neeraj.Upadhyay@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v2 1/3] rcu: Reduce synchronize_rcu() latency

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@...il.com>

This looks pretty close!  Some questions and comments below, much of
which being what I managed not to write down in earlier discussions.  :-/

> ---
>  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.  ;-)

> +/* 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.

> +	/* 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/

> + */
> +#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;

> +		}
> +	}
> +
> +	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?

> +	/*
> +	 * 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.

> +}
> +
> +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/?

> +}
> +
>  /*
>   * 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.

> +}
> +
>  /**
>   * synchronize_rcu - wait until a grace period has elapsed.
>   *
> @@ -3551,7 +3703,7 @@ void synchronize_rcu(void)
>  		if (rcu_gp_is_expedited())
>  			synchronize_rcu_expedited();
>  		else
> -			wait_rcu_gp(call_rcu_hurry);
> +			synchronize_rcu_normal();
>  		return;
>  	}
>  
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 6d7cea5d591f..279a37beb05a 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -987,7 +987,7 @@ void synchronize_rcu_expedited(void)
>  
>  	/* If expedited grace periods are prohibited, fall back to normal. */
>  	if (rcu_gp_is_normal()) {
> -		wait_rcu_gp(call_rcu_hurry);
> +		synchronize_rcu_normal();
>  		return;
>  	}
>  
> -- 
> 2.30.2
> 

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