Re: [PATCH v2 1/1] rcu: Reduce synchronize_rcu() waiting time

From: Paul E. McKenney
Date: Wed Oct 04 2023 - 11:24:39 EST

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On Tue, Oct 03, 2023 at 08:04:03PM +0200, Uladzislau Rezki (Sony) wrote:
> A call to a synchronize_rcu() can be optimized from time point of
> view. Different workloads can be affected by this especially the
> ones which use this API in its time critical sections.
>
> For example if CONFIG_RCU_NOCB_CPU is set, the wakeme_after_rcu()
> callback can be delayed and such delay depends on where in a nocb
> list it is located.
>
> 1. On our Android 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. On our Android devices 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" that
> binds to all CPUs, i.e. tasks can be migrated between groups.
>
> See below an example of how "surfaceflinger" task is migrated.
> Initially it is located in the "system-background" cgroup which
> allows to run only on little cores. In order to speedup it up
> it can be temporary moved into "foreground" cgroup which allows
> to use big 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
> PERFD-SERVER-1855 [000] d..1. 7030.530383: 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>
>
> from this example it is clear that "a moving time" also depends
> on how fast synchronize_rcu() completes.
>
> 3. This patch improves the synchronize_rcu() approximately by 30%-50%
> on synthetic tests. Apart of that i have tested app launch of camera
> app where i also see better perf. figures:
>
> 542 vs 489 diff: 9%
> 540 vs 466 diff: 13%
> 518 vs 468 diff: 9%
> 531 vs 457 diff: 13%
> 548 vs 475 diff: 13%
> 509 vs 484 diff: 4%
>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> ---
> kernel/rcu/tree.c | 151 +++++++++++++++++++++++++++++++++++++++++-
> kernel/rcu/tree_exp.h | 2 +-
> 2 files changed, 151 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 78554e7181dd..a347c1f98f11 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1384,6 +1384,122 @@ 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 curr; /* request a GP users. */
> + struct llist_head wait; /* wait for GP users. */
> + struct llist_head done; /* ready for GP users. */
> + struct llist_node *curr_tail;
> + struct llist_node *wait_tail;
> + atomic_t active;
> +} sr;
> +
> +/* Enable it by default. */
> +static int rcu_normal_wake_from_gp = 1;
> +module_param(rcu_normal_wake_from_gp, int, 0644);

Nice!

But could you please make this default to zero in order to avoid
surprising people for whom the old way works better?

Thanx, Paul

> +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;
> +
> + if (!poll_state_synchronize_rcu(oldstate))
> + WARN_ONCE(1, "A full grace period is not passed yet: %lu",
> + rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
> +
> + /* 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.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);
> +
> +/*
> + * Helper function for rcu_gp_cleanup().
> + */
> +static void rcu_sr_normal_gp_cleanup(void)
> +{
> + struct llist_node *first, *tail;
> +
> + tail = READ_ONCE(sr.wait_tail);
> + first = llist_del_all(&sr.wait);
> + if (!first)
> + return;
> +
> + /* Only one user? */
> + if (!first->next) {
> + rcu_sr_normal_complete(first);
> + return;
> + }
> +
> + /* Can be not empty. */
> + llist_add_batch(first, tail, &sr.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 *llnode, *rcu;
> + int ret;
> +
> + if (llist_empty(&sr.curr))
> + return;
> +
> + /*
> + * 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.wait));
> +
> + /*
> + * Obtain a tail of current active users. It is guaranteed
> + * that if we are only one active user and the list is not
> + * empty, the tail has already been updated.
> + */
> + ret = atomic_inc_return(&sr.active);
> + WRITE_ONCE(sr.wait_tail, (ret == 1) ? READ_ONCE(sr.curr_tail):NULL);
> + llnode = llist_del_all(&sr.curr);
> + atomic_dec(&sr.active);
> +
> + if (ret != 1) {
> + llist_for_each(rcu, llnode) {
> + if (!rcu->next)
> + WRITE_ONCE(sr.wait_tail, rcu);
> + }
> + }
> +
> + llist_add_batch(llnode, READ_ONCE(sr.wait_tail), &sr.wait);
> +}
> +
> +static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> +{
> + atomic_inc(&sr.active);
> + if (llist_add((struct llist_node *) &rs->head, &sr.curr))
> + /* Set the tail. Only first and one user can do that. */
> + WRITE_ONCE(sr.curr_tail, (struct llist_node *) &rs->head);
> + atomic_dec(&sr.active);
> +}
> +
> /*
> * Initialize a new grace period. Return false if no grace period required.
> */
> @@ -1420,6 +1536,7 @@ static noinline_for_stack bool rcu_gp_init(void)
> ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
> trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
> rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
> + rcu_sr_normal_gp_init();
> raw_spin_unlock_irq_rcu_node(rnp);
>
> /*
> @@ -1787,6 +1904,9 @@ 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 +3620,35 @@ 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);
> + }
> +}
> +
> /**
> * synchronize_rcu - wait until a grace period has elapsed.
> *
> @@ -3551,7 +3700,7 @@ 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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In reply to: Uladzislau Rezki (Sony): "[PATCH v2 1/1] rcu: Reduce synchronize_rcu() waiting time"
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