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SubjectRe: [PATCH v3] erofs: replace erofs_unzipd workqueue with per-cpu threads
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Hi Sandeep,

On 2023/1/6 13:41, Sandeep Dhavale wrote:
> Using per-cpu thread pool we can reduce the scheduling latency compared
> to workqueue implementation. With this patch scheduling latency and
> variation is reduced as per-cpu threads are high priority kthread_workers.
>
> The results were evaluated on arm64 Android devices running 5.10 kernel.
>
> The table below shows resulting improvements of total scheduling latency
> for the same app launch benchmark runs with 50 iterations. Scheduling
> latency is the latency between when the task (workqueue kworker vs
> kthread_worker) became eligible to run to when it actually started
> running.
> +-------------------------+-----------+----------------+---------+
> | | workqueue | kthread_worker | diff |
> +-------------------------+-----------+----------------+---------+
> | Average (us) | 15253 | 2914 | -80.89% |
> | Median (us) | 14001 | 2912 | -79.20% |
> | Minimum (us) | 3117 | 1027 | -67.05% |
> | Maximum (us) | 30170 | 3805 | -87.39% |
> | Standard deviation (us) | 7166 | 359 | |
> +-------------------------+-----------+----------------+---------+
>
> Signed-off-by: Sandeep Dhavale <dhavale@google.com>
> ---
> V2 -> V3
> Fix a warning Reported-by: kernel test robot <lkp@intel.com>
>
> V1 -> V2
> * Changed name of kthread_workers from z_erofs to erofs_worker
> * Added kernel configuration to run kthread_workers at normal or
> high priority
> * Added cpu hotplug support
> * Added wrapped kthread_workers under worker_pool
> * Added one unbound thread in a pool to handle a context where
> we already stopped per-cpu kthread worker
> * Updated commit message
>
> Background: Boot times and cold app launch benchmarks are very
> important to android ecosystem as they directly translate to
> responsiveness from user point of view. While erofs provides
> lot of important features like space savings, we saw some
> performance penalty in cold app launch benchmarks in few scenarios.
> Analysis showed that the significant variance was coming from the
> scheduling cost while decompression cost was more or less the same.
>
> Having per-cpu thread pool we can see from above table that this
> variation is reduced by ~80% on average. Link to LPC 2022 slides and
> talk at [1]
>
> [1] https://lpc.events/event/16/contributions/1338/

Let's move these words into the commit message as well for others to refer.

I think look into (and try) this version later this week.

Thanks,
Gao Xiang

> ---
> fs/erofs/Kconfig | 11 +++
> fs/erofs/zdata.c | 201 +++++++++++++++++++++++++++++++++++++++++------
> fs/erofs/zdata.h | 4 +-
> 3 files changed, 192 insertions(+), 24 deletions(-)
>
> diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
> index 85490370e0ca..879f493c6641 100644
> --- a/fs/erofs/Kconfig
> +++ b/fs/erofs/Kconfig
> @@ -108,3 +108,14 @@ config EROFS_FS_ONDEMAND
> read support.
>
> If unsure, say N.
> +
> +config EROFS_FS_KTHREAD_HIPRI
> + bool "EROFS high priority percpu kthread workers"
> + depends on EROFS_FS
> + default n
> + help
> + EROFS uses per cpu kthread workers pool to carry out async work.
> + This permits EROFS to configure per cpu kthread workers to run
> + at higher priority.
> +
> + If unsure, say N.
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index ccf7c55d477f..b4bf2da72d1a 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -8,6 +8,8 @@
> #include "compress.h"
> #include <linux/prefetch.h>
> #include <linux/psi.h>
> +#include <linux/slab.h>
> +#include <linux/cpuhotplug.h>
>
> #include <trace/events/erofs.h>
>
> @@ -184,26 +186,152 @@ typedef tagptr1_t compressed_page_t;
> #define tag_compressed_page_justfound(page) \
> tagptr_fold(compressed_page_t, page, 1)
>
> -static struct workqueue_struct *z_erofs_workqueue __read_mostly;
> +struct erofs_kthread_worker_pool {
> + struct kthread_worker __rcu **workers;
> + struct kthread_worker *unbound_worker;
> +};
>
> -void z_erofs_exit_zip_subsystem(void)
> +static struct erofs_kthread_worker_pool worker_pool;
> +DEFINE_SPINLOCK(worker_pool_lock);
> +
> +static void erofs_destroy_worker_pool(void)
> {
> - destroy_workqueue(z_erofs_workqueue);
> - z_erofs_destroy_pcluster_pool();
> + unsigned int cpu;
> + struct kthread_worker *worker;
> +
> + for_each_possible_cpu(cpu) {
> + worker = rcu_dereference_protected(
> + worker_pool.workers[cpu],
> + 1);
> + rcu_assign_pointer(worker_pool.workers[cpu], NULL);
> +
> + if (worker)
> + kthread_destroy_worker(worker);
> + }
> +
> + if (worker_pool.unbound_worker)
> + kthread_destroy_worker(worker_pool.unbound_worker);
> +
> + kfree(worker_pool.workers);
> }
>
> -static inline int z_erofs_init_workqueue(void)
> +static inline void erofs_set_worker_priority(struct kthread_worker *worker)
> {
> - const unsigned int onlinecpus = num_possible_cpus();
> +#ifdef CONFIG_EROFS_FS_KTHREAD_HIPRI
> + sched_set_fifo_low(worker->task);
> +#else
> + sched_set_normal(worker->task, 0);
> +#endif
> +}
>
> - /*
> - * no need to spawn too many threads, limiting threads could minimum
> - * scheduling overhead, perhaps per-CPU threads should be better?
> - */
> - z_erofs_workqueue = alloc_workqueue("erofs_unzipd",
> - WQ_UNBOUND | WQ_HIGHPRI,
> - onlinecpus + onlinecpus / 4);
> - return z_erofs_workqueue ? 0 : -ENOMEM;
> +static int erofs_create_kthread_workers(void)
> +{
> + unsigned int cpu;
> + struct kthread_worker *worker;
> +
> + for_each_online_cpu(cpu) {
> + worker = kthread_create_worker_on_cpu(cpu, 0, "erofs_worker/%u", cpu);
> + if (IS_ERR(worker)) {
> + erofs_destroy_worker_pool();
> + return -ENOMEM;
> + }
> + erofs_set_worker_priority(worker);
> + rcu_assign_pointer(worker_pool.workers[cpu], worker);
> + }
> +
> + worker = kthread_create_worker(0, "erofs_unbound_worker");
> + if (IS_ERR(worker)) {
> + erofs_destroy_worker_pool();
> + return PTR_ERR(worker);
> + }
> + erofs_set_worker_priority(worker);
> + worker_pool.unbound_worker = worker;
> +
> + return 0;
> +}
> +
> +static int erofs_init_worker_pool(void)
> +{
> + int err;
> +
> + worker_pool.workers = kcalloc(num_possible_cpus(),
> + sizeof(struct kthread_worker *), GFP_ATOMIC);
> + if (!worker_pool.workers)
> + return -ENOMEM;
> + err = erofs_create_kthread_workers();
> +
> + return err;
> +}
> +
> +#ifdef CONFIG_HOTPLUG_CPU
> +static enum cpuhp_state erofs_cpuhp_state;
> +static int erofs_cpu_online(unsigned int cpu)
> +{
> + struct kthread_worker *worker;
> +
> + worker = kthread_create_worker_on_cpu(cpu, 0, "erofs_worker/%u", cpu);
> + if (IS_ERR(worker))
> + return -ENOMEM;
> +
> + erofs_set_worker_priority(worker);
> +
> + spin_lock(&worker_pool_lock);
> + rcu_assign_pointer(worker_pool.workers[cpu], worker);
> + spin_unlock(&worker_pool_lock);
> +
> + synchronize_rcu();
> +
> + return 0;
> +}
> +
> +static int erofs_cpu_offline(unsigned int cpu)
> +{
> + struct kthread_worker *worker;
> +
> + spin_lock(&worker_pool_lock);
> + worker = rcu_dereference_protected(worker_pool.workers[cpu],
> + lockdep_is_held(&worker_pool_lock));
> + rcu_assign_pointer(worker_pool.workers[cpu], NULL);
> + spin_unlock(&worker_pool_lock);
> +
> + synchronize_rcu();
> +
> + if (worker)
> + kthread_destroy_worker(worker);
> +
> + return 0;
> +}
> +
> +static int erofs_cpu_hotplug_init(void)
> +{
> + int state;
> +
> + state = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> + "fs/erofs:online",
> + erofs_cpu_online, erofs_cpu_offline);
> + if (state < 0)
> + return state;
> +
> + erofs_cpuhp_state = state;
> +
> + return 0;
> +}
> +
> +static void erofs_cpu_hotplug_destroy(void)
> +{
> + if (erofs_cpuhp_state)
> + cpuhp_remove_state_nocalls(erofs_cpuhp_state);
> +}
> +#else /* !CONFIG_HOTPLUG_CPU */
> +static inline int erofs_cpu_hotplug_init(void) { return 0; }
> +static inline void erofs_cpu_hotplug_destroy(void) {}
> +#endif
> +
> +void z_erofs_exit_zip_subsystem(void)
> +{
> + erofs_cpu_hotplug_destroy();
> + erofs_destroy_worker_pool();
> + z_erofs_destroy_pcluster_pool();
> }
>
> int __init z_erofs_init_zip_subsystem(void)
> @@ -211,10 +339,23 @@ int __init z_erofs_init_zip_subsystem(void)
> int err = z_erofs_create_pcluster_pool();
>
> if (err)
> - return err;
> - err = z_erofs_init_workqueue();
> + goto out_error_pcluster_pool;
> +
> + err = erofs_init_worker_pool();
> if (err)
> - z_erofs_destroy_pcluster_pool();
> + goto out_error_worker_pool;
> +
> + err = erofs_cpu_hotplug_init();
> + if (err < 0)
> + goto out_error_cpuhp_init;
> +
> + return err;
> +
> +out_error_cpuhp_init:
> + erofs_destroy_worker_pool();
> +out_error_worker_pool:
> + z_erofs_destroy_pcluster_pool();
> +out_error_pcluster_pool:
> return err;
> }
>
> @@ -1143,7 +1284,7 @@ static void z_erofs_decompress_queue(const struct z_erofs_decompressqueue *io,
> }
> }
>
> -static void z_erofs_decompressqueue_work(struct work_struct *work)
> +static void z_erofs_decompressqueue_kthread_work(struct kthread_work *work)
> {
> struct z_erofs_decompressqueue *bgq =
> container_of(work, struct z_erofs_decompressqueue, u.work);
> @@ -1156,6 +1297,20 @@ static void z_erofs_decompressqueue_work(struct work_struct *work)
> kvfree(bgq);
> }
>
> +static void erofs_schedule_kthread_work(struct kthread_work *work)
> +{
> + struct kthread_worker *worker;
> + unsigned int cpu = raw_smp_processor_id();
> +
> + rcu_read_lock();
> + worker = rcu_dereference(worker_pool.workers[cpu]);
> + if (!worker)
> + worker = worker_pool.unbound_worker;
> +
> + kthread_queue_work(worker, work);
> + rcu_read_unlock();
> +}
> +
> static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
> bool sync, int bios)
> {
> @@ -1170,15 +1325,15 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
>
> if (atomic_add_return(bios, &io->pending_bios))
> return;
> - /* Use workqueue and sync decompression for atomic contexts only */
> + /* Use kthread_workers and sync decompression for atomic contexts only */
> if (in_atomic() || irqs_disabled()) {
> - queue_work(z_erofs_workqueue, &io->u.work);
> + erofs_schedule_kthread_work(&io->u.work);
> /* enable sync decompression for readahead */
> if (sbi->opt.sync_decompress == EROFS_SYNC_DECOMPRESS_AUTO)
> sbi->opt.sync_decompress = EROFS_SYNC_DECOMPRESS_FORCE_ON;
> return;
> }
> - z_erofs_decompressqueue_work(&io->u.work);
> + z_erofs_decompressqueue_kthread_work(&io->u.work);
> }
>
> static struct page *pickup_page_for_submission(struct z_erofs_pcluster *pcl,
> @@ -1306,7 +1461,7 @@ jobqueue_init(struct super_block *sb,
> *fg = true;
> goto fg_out;
> }
> - INIT_WORK(&q->u.work, z_erofs_decompressqueue_work);
> + kthread_init_work(&q->u.work, z_erofs_decompressqueue_kthread_work);
> } else {
> fg_out:
> q = fgq;
> @@ -1500,7 +1655,7 @@ static void z_erofs_submit_queue(struct z_erofs_decompress_frontend *f,
>
> /*
> * although background is preferred, no one is pending for submission.
> - * don't issue workqueue for decompression but drop it directly instead.
> + * don't issue kthread_work for decompression but drop it directly instead.
> */
> if (!*force_fg && !nr_bios) {
> kvfree(q[JQ_SUBMIT]);
> diff --git a/fs/erofs/zdata.h b/fs/erofs/zdata.h
> index d98c95212985..808bbbf71b7b 100644
> --- a/fs/erofs/zdata.h
> +++ b/fs/erofs/zdata.h
> @@ -6,6 +6,8 @@
> #ifndef __EROFS_FS_ZDATA_H
> #define __EROFS_FS_ZDATA_H
>
> +#include <linux/kthread.h>
> +
> #include "internal.h"
> #include "tagptr.h"
>
> @@ -107,7 +109,7 @@ struct z_erofs_decompressqueue {
>
> union {
> struct completion done;
> - struct work_struct work;
> + struct kthread_work work;
> } u;
>
> bool eio;

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