On Thu, May 12, 2022 at 03:04:29AM +0000, Joel Fernandes (Google) wrote:
> Implement timer-based RCU callback batching. The batch is flushed
> whenever a certain amount of time has passed, or the batch on a
> particular CPU grows too big. Also memory pressure can flush it.
>
> Locking is avoided to reduce lock contention when queuing and dequeuing
> happens on different CPUs of a per-cpu list, such as when shrinker
> context is running on different CPU. Also not having to use locks keeps
> the per-CPU structure size small.
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
It is very good to see this! Inevitable comments and questions below.
Thanx, Paul
> ---
> include/linux/rcupdate.h | 6 ++
> kernel/rcu/Kconfig | 8 +++
> kernel/rcu/Makefile | 1 +
> kernel/rcu/lazy.c | 145 +++++++++++++++++++++++++++++++++++++++
> kernel/rcu/rcu.h | 5 ++
> kernel/rcu/tree.c | 2 +
> 6 files changed, 167 insertions(+)
> create mode 100644 kernel/rcu/lazy.c
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 88b42eb46406..d0a6c4f5172c 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -82,6 +82,12 @@ static inline int rcu_preempt_depth(void)
>
> #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
>
> +#ifdef CONFIG_RCU_LAZY
> +void call_rcu_lazy(struct rcu_head *head, rcu_callback_t func);
> +#else
> +#define call_rcu_lazy(head, func) call_rcu(head, func)
> +#endif
> +
> /* Internal to kernel */
> void rcu_init(void);
> extern int rcu_scheduler_active __read_mostly;
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index bf8e341e75b4..c09715079829 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -241,4 +241,12 @@ config TASKS_TRACE_RCU_READ_MB
> Say N here if you hate read-side memory barriers.
> Take the default if you are unsure.
>
> +config RCU_LAZY
> + bool "RCU callback lazy invocation functionality"
> + depends on RCU_NOCB_CPU
> + default y
This "default y" is OK for experimentation, but for mainline we should
not be forcing this on unsuspecting people. ;-)
> + help
> + To save power, batch RCU callbacks and flush after delay, memory
> + pressure or callback list growing too big.
> +
> endmenu # "RCU Subsystem"
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 0cfb009a99b9..8968b330d6e0 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -16,3 +16,4 @@ obj-$(CONFIG_RCU_REF_SCALE_TEST) += refscale.o
> obj-$(CONFIG_TREE_RCU) += tree.o
> obj-$(CONFIG_TINY_RCU) += tiny.o
> obj-$(CONFIG_RCU_NEED_SEGCBLIST) += rcu_segcblist.o
> +obj-$(CONFIG_RCU_LAZY) += lazy.o
> diff --git a/kernel/rcu/lazy.c b/kernel/rcu/lazy.c
> new file mode 100644
> index 000000000000..55e406cfc528
> --- /dev/null
> +++ b/kernel/rcu/lazy.c
> @@ -0,0 +1,145 @@
> +/*
> + * Lockless lazy-RCU implementation.
> + */
> +#include <linux/rcupdate.h>
> +#include <linux/shrinker.h>
> +#include <linux/workqueue.h>
> +#include "rcu.h"
> +
> +// How much to batch before flushing?
> +#define MAX_LAZY_BATCH 2048
> +
> +// How much to wait before flushing?
> +#define MAX_LAZY_JIFFIES 10000
That is more than a minute on a HZ=10 system. Are you sure that you
did not mean "(10 * HZ)" or some such?
> +
> +// We cast lazy_rcu_head to rcu_head and back. This keeps the API simple while
> +// allowing us to use lockless list node in the head. Also, we use BUILD_BUG_ON
> +// later to ensure that rcu_head and lazy_rcu_head are of the same size.
> +struct lazy_rcu_head {
> + struct llist_node llist_node;
> + void (*func)(struct callback_head *head);
> +} __attribute__((aligned(sizeof(void *))));
This needs a build-time check that rcu_head and lazy_rcu_head are of
the same size. Maybe something like this in some appropriate context:
BUILD_BUG_ON(sizeof(struct rcu_head) != sizeof(struct_rcu_head_lazy));
Never mind! I see you have this in rcu_init_lazy(). Plus I now see that
you also mention this in the above comments. ;-)
> +
> +struct rcu_lazy_pcp {
> + struct llist_head head;
> + struct delayed_work work;
> + atomic_t count;
> +};
> +DEFINE_PER_CPU(struct rcu_lazy_pcp, rcu_lazy_pcp_ins);
> +
> +// Lockless flush of CPU, can be called concurrently.
> +static void lazy_rcu_flush_cpu(struct rcu_lazy_pcp *rlp)
> +{
> + struct llist_node *node = llist_del_all(&rlp->head);
> + struct lazy_rcu_head *cursor, *temp;
> +
> + if (!node)
> + return;
At this point, the list is empty but the count is non-zero. Can
that cause a problem? (For the existing callback lists, this would
be OK.)
> + llist_for_each_entry_safe(cursor, temp, node, llist_node) {
> + struct rcu_head *rh = (struct rcu_head *)cursor;
> + debug_rcu_head_unqueue(rh);
Good to see this check!
> + call_rcu(rh, rh->func);
> + atomic_dec(&rlp->count);
> + }
> +}
> +
> +void call_rcu_lazy(struct rcu_head *head_rcu, rcu_callback_t func)
> +{
> + struct lazy_rcu_head *head = (struct lazy_rcu_head *)head_rcu;
> + struct rcu_lazy_pcp *rlp;
> +
> + preempt_disable();
> + rlp = this_cpu_ptr(&rcu_lazy_pcp_ins);
Whitespace issue, please fix.
> + preempt_enable();
> +
> + if (debug_rcu_head_queue((void *)head)) {
> + // Probable double call_rcu(), just leak.
> + WARN_ONCE(1, "%s(): Double-freed call. rcu_head %p\n",
> + __func__, head);
> +
> + // Mark as success and leave.
> + return;
> + }
> +
> + // Queue to per-cpu llist
> + head->func = func;
> + llist_add(&head->llist_node, &rlp->head);
Suppose that there are a bunch of preemptions between the preempt_enable()
above and this point, so that the current CPU's list has lots of
callbacks, but zero ->count. Can that cause a problem?
In the past, this sort of thing has been an issue for rcu_barrier()
and friends.
> + // Flush queue if too big
You will also need to check for early boot use.
I -think- it suffice to simply skip the following "if" statement when
rcu_scheduler_active == RCU_SCHEDULER_INACTIVE suffices. The reason being
that you can queue timeouts at that point, even though CONFIG_PREEMPT_RT
kernels won't expire them until the softirq kthreads have been spawned.
Which is OK, as it just means that call_rcu_lazy() is a bit more
lazy than expected that early.
Except that call_rcu() can be invoked even before rcu_init() has been
invoked, which is therefore also before rcu_init_lazy() has been invoked.
I thefore suggest something like this at the very start of this function:
if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
call_rcu(head_rcu, func);
The goal is that people can replace call_rcu() with call_rcu_lazy()
without having to worry about invocation during early boot.
Huh. "head_rcu"? Why not "rhp"? Or follow call_rcu() with "head",
though "rhp" is more consistent with the RCU pointer initials approach.
> + if (atomic_inc_return(&rlp->count) >= MAX_LAZY_BATCH) {
> + lazy_rcu_flush_cpu(rlp);
> + } else {
> + if (!delayed_work_pending(&rlp->work)) {
This check is racy because the work might run to completion right at
this point. Wouldn't it be better to rely on the internal check of
WORK_STRUCT_PENDING_BIT within queue_delayed_work_on()?
> + schedule_delayed_work(&rlp->work, MAX_LAZY_JIFFIES);
> + }
> + }
> +}
> +
> +static unsigned long
> +lazy_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> +{
> + unsigned long count = 0;
> + int cpu;
> +
> + /* Snapshot count of all CPUs */
> + for_each_possible_cpu(cpu) {
> + struct rcu_lazy_pcp *rlp = per_cpu_ptr(&rcu_lazy_pcp_ins, cpu);
> +
> + count += atomic_read(&rlp->count);
> + }
> +
> + return count;
> +}
> +
> +static unsigned long
> +lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> +{
> + int cpu, freed = 0;
> +
> + for_each_possible_cpu(cpu) {
> + struct rcu_lazy_pcp *rlp = per_cpu_ptr(&rcu_lazy_pcp_ins, cpu);
> + unsigned long count;
> +
> + count = atomic_read(&rlp->count);
> + lazy_rcu_flush_cpu(rlp);
> + sc->nr_to_scan -= count;
> + freed += count;
> + if (sc->nr_to_scan <= 0)
> + break;
> + }
> +
> + return freed == 0 ? SHRINK_STOP : freed;
This is a bit surprising given the stated aim of SHRINK_STOP to indicate
potential deadlocks. But this pattern is common, including on the
kvfree_rcu() path, so OK! ;-)
Plus do_shrink_slab() loops if you don't return SHRINK_STOP, so there is
that as well.
> +}
> +
> +/*
> + * This function is invoked after MAX_LAZY_JIFFIES timeout.
> + */
> +static void lazy_work(struct work_struct *work)
> +{
> + struct rcu_lazy_pcp *rlp = container_of(work, struct rcu_lazy_pcp, work.work);
> +
> + lazy_rcu_flush_cpu(rlp);
> +}
> +
> +static struct shrinker lazy_rcu_shrinker = {
> + .count_objects = lazy_rcu_shrink_count,
> + .scan_objects = lazy_rcu_shrink_scan,
> + .batch = 0,
> + .seeks = DEFAULT_SEEKS,
> +};
> +
> +void __init rcu_lazy_init(void)
> +{
> + int cpu;
> +
> + BUILD_BUG_ON(sizeof(struct lazy_rcu_head) != sizeof(struct rcu_head));
> +
> + for_each_possible_cpu(cpu) {
> + struct rcu_lazy_pcp *rlp = per_cpu_ptr(&rcu_lazy_pcp_ins, cpu);
> + INIT_DELAYED_WORK(&rlp->work, lazy_work);
> + }
> +
> + if (register_shrinker(&lazy_rcu_shrinker))
> + pr_err("Failed to register lazy_rcu shrinker!\n");
> +}
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 24b5f2c2de87..a5f4b44f395f 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -561,4 +561,9 @@ void show_rcu_tasks_trace_gp_kthread(void);
> static inline void show_rcu_tasks_trace_gp_kthread(void) {}
> #endif
>
> +#ifdef CONFIG_RCU_LAZY
> +void rcu_lazy_init(void);
> +#else
> +static inline void rcu_lazy_init(void) {}
> +#endif
> #endif /* __LINUX_RCU_H */
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a4c25a6283b0..ebdf6f7c9023 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4775,6 +4775,8 @@ void __init rcu_init(void)
> qovld_calc = DEFAULT_RCU_QOVLD_MULT * qhimark;
> else
> qovld_calc = qovld;
> +
> + rcu_lazy_init();
> }
>
> #include "tree_stall.h"
> --
> 2.36.0.550.gb090851708-goog
>
