On Thu, Jan 04, 2024 at 05:25:07PM +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 deferred to a dedicated worker to process requests.
>
> 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(no "noise" from other callbacks):
> Hardware: x86_64 64 CPUs, 64GB of memory
> Linux-6.6
>
> - 10K tasks(simultaneous);
> - each task does(1000 loops)
> synchronize_rcu();
> kfree(p);
>
> default: CONFIG_RCU_NOCB_CPU: takes 54 seconds to complete all users;
> patch: CONFIG_RCU_NOCB_CPU: takes 35 seconds to complete all users.
>
> Running 60K gives approximately same results on my setup. Please note
> it is without any interaction with another type of callbacks, otherwise
> it will impact a lot a default case.
>
> 5. An extra CONFIG_RCU_SR_NORMAL_DEBUG_GP kernel option is added
> which enables additional debugging for detecting a grace period
> incompletion for synchronize_rcu() users. If a GP is not fully
> passed for any user, the warning message is emitted.
>
> 6. By default it is disabled. To enable this perform one of the
> below sequence:
>
> echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> or pass a boot parameter "rcutree.rcu_normal_wake_from_gp=1"
>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Again, nice latency reductions!
A few comments and questions below.
Thanx, Paul
> ---
> .../admin-guide/kernel-parameters.txt | 14 ++
> kernel/rcu/Kconfig.debug | 12 ++
> kernel/rcu/tree.c | 138 +++++++++++++++++-
> kernel/rcu/tree_exp.h | 2 +-
> 4 files changed, 164 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 17a454909ab4..2cca75e4f0c6 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5047,6 +5047,20 @@
> delay, memory pressure or callback list growing too
> big.
>
> + rcutree.rcu_normal_wake_from_gp= [KNL]
> + Reduces a latency of synchronize_rcu() call. This approach
> + maintains its own track of synchronize_rcu() callers, so it
> + does not interact with regular callbacks because it does not
> + use a call_rcu[_hurry]() path. Please note, this is for a
> + normal grace period.
> +
> + How to enable it:
> +
> + echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> + or pass a boot parameter "rcutree.rcu_normal_wake_from_gp=1"
> +
> + Default is 0.
> +
> rcuscale.gp_async= [KNL]
> Measure performance of asynchronous
> grace-period primitives such as call_rcu().
> diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
> index 9b0b52e1836f..4812c6249185 100644
> --- a/kernel/rcu/Kconfig.debug
> +++ b/kernel/rcu/Kconfig.debug
> @@ -168,4 +168,16 @@ config RCU_STRICT_GRACE_PERIOD
> when looking for certain types of RCU usage bugs, for example,
> too-short RCU read-side critical sections.
>
> +config RCU_SR_NORMAL_DEBUG_GP
> + bool "Debug synchronize_rcu() callers for a grace period completion"
> + depends on DEBUG_KERNEL && RCU_EXPERT
> + default n
> + help
> + This option enables additional debugging for detecting a grace
> + period incompletion for synchronize_rcu() users. If a GP is not
> + fully passed for any user, the warning message is emitted.
> +
> + Say Y here if you want to enable such debugging
> + Say N if you are unsure.
> +
> endmenu # "RCU Debugging"
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 499803234176..b756c40e4960 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1422,6 +1422,106 @@ 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;
Please put this in the rcu_state structure. Having the separate structure
is fine (it does group the fields nicely, plus you can take a pointer
to it in the functions using this state), but it is good to have the
state in one place.
Also, please add the data structures in a separate patch. This might
save someone a lot of time and effort should someone breaks the kernel
in a way that depends on data-structure size. It would be much easier
for us if their bisection converged on the commit that adds the data
structures instead of the commit that also adds a lot of code.
> +
> +/* 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(IS_ENABLED(CONFIG_RCU_SR_NORMAL_DEBUG_GP) &&
> + !poll_state_synchronize_rcu(oldstate),
> + "A full grace period is not passed yet: %lu",
> + rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
Good, the false-positive-prone check is now under debug. Or at
least possible, even if not prone.
> + /* 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);
Why not put this into the sr_normal_state structure? You can use
__WORK_INITIALIZER() to initialize it, as is done in a number of other
places in the kernel.
> +/*
> + * Helper function for rcu_gp_cleanup().
> + */
> +static void rcu_sr_normal_gp_cleanup(void)
> +{
> + struct llist_node *head, *tail;
> +
> + if (llist_empty(&sr.srs_wait))
> + return;
> +
> + tail = READ_ONCE(sr.srs_wait_tail);
> + head = __llist_del_all(&sr.srs_wait);
> +
> + 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);
Again, reversing the order is going to cause trouble on large systems.
Let's please not do that. (I could have sworn that this was not present
in the last series...)
> + /*
> + * 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);
> +}
> +
> +static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> +{
> + llist_add((struct llist_node *) &rs->head, &sr.srs_next);
> +}
> +
> /*
> * Initialize a new grace period. Return false if no grace period required.
> */
> @@ -1456,6 +1556,7 @@ 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);
> @@ -1825,6 +1926,9 @@ 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);
> @@ -3561,6 +3665,38 @@ 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)) {
> + wait_rcu_gp(call_rcu_hurry);
> + return;
> + }
> +
> + 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.
> + */
> + if (IS_ENABLED(CONFIG_RCU_SR_NORMAL_DEBUG_GP))
> + 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();
It is unfortunate that the debugging requires an extra timestamp.
The ways I can think of to avoid this have problems, though. If this
thing was replicated per leaf rcu_node structure, the usual approach
would be to protect it with that structure's ->lock.
Thoughts?
> + /* Now we can wait. */
> + wait_for_completion(&rs.completion);
> + destroy_rcu_head_on_stack(&rs.head);
> +}
> +
> /**
> * synchronize_rcu - wait until a grace period has elapsed.
> *
> @@ -3612,7 +3748,7 @@ 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 014ddf672165..bdc30d972d32 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -985,7 +985,7 @@ 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;
> }
>
> --
> 2.39.2
>
