[PATCH v5] erofs: add per-cpu threads for decompression as an option

[Gao Xiang]

From: Sandeep Dhavale <dhavale@xxxxxxxxxx>

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 |         |
+-------------------------+-----------+----------------+---------+

Background: Boot times and cold app launch benchmarks are very
important to the Android ecosystem as they directly translate to
responsiveness from user point of view. While EROFS provides
a 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 the above table that this
variation is reduced by ~80% on average. This problem was discussed
at LPC 2022. Link to LPC 2022 slides and talk at [1]

[1] https://lpc.events/event/16/contributions/1338/

Signed-off-by: Sandeep Dhavale <dhavale@xxxxxxxxxx>
[ Gao Xiang: At least, we have to add this until WQ_UNBOUND workqueue
             issue [2] on many arm64 devices is resolved. ]
[2] https://lore.kernel.org/r/CAJkfWY490-m6wNubkxiTPsW59sfsQs37Wey279LmiRxKt7aQYg@xxxxxxxxxxxxxx
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
---
v4: https://lore.kernel.org/r/20230106073502.4017276-1-dhavale@xxxxxxxxxx/

changes since v4:
 - need to fall back to workqueue rather than a global kthread and
   make this feature as an option for now.

 fs/erofs/Kconfig |  18 +++++
 fs/erofs/zdata.c | 190 ++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 188 insertions(+), 20 deletions(-)

diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
index 85490370e0ca..704fb59577e0 100644
--- a/fs/erofs/Kconfig
+++ b/fs/erofs/Kconfig
@@ -108,3 +108,21 @@ config EROFS_FS_ONDEMAND
 	  read support.
 
 	  If unsure, say N.
+
+config EROFS_FS_PCPU_KTHREAD
+	bool "EROFS per-cpu decompression kthread workers"
+	depends on EROFS_FS_ZIP
+	help
+	  Saying Y here enables per-CPU kthread workers pool to carry out
+	  async decompression for low latencies on some architectures.
+
+	  If unsure, say N.
+
+config EROFS_FS_PCPU_KTHREAD_HIPRI
+	bool "EROFS high priority per-CPU kthread workers"
+	depends on EROFS_FS_ZIP && EROFS_FS_PCPU_KTHREAD
+	help
+	  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 384f64292f73..8ea3f5fe985e 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -7,7 +7,7 @@
 #include "compress.h"
 #include <linux/prefetch.h>
 #include <linux/psi.h>
-
+#include <linux/cpuhotplug.h>
 #include <trace/events/erofs.h>
 
 #define Z_EROFS_PCLUSTER_MAX_PAGES	(Z_EROFS_PCLUSTER_MAX_SIZE / PAGE_SIZE)
@@ -109,6 +109,7 @@ struct z_erofs_decompressqueue {
 	union {
 		struct completion done;
 		struct work_struct work;
+		struct kthread_work kthread_work;
 	} u;
 	bool eio, sync;
 };
@@ -341,24 +342,128 @@ static void z_erofs_free_pcluster(struct z_erofs_pcluster *pcl)
 
 static struct workqueue_struct *z_erofs_workqueue __read_mostly;
 
-void z_erofs_exit_zip_subsystem(void)
+#ifdef CONFIG_EROFS_FS_PCPU_KTHREAD
+static struct kthread_worker __rcu **z_erofs_pcpu_workers;
+
+static void erofs_destroy_percpu_workers(void)
 {
-	destroy_workqueue(z_erofs_workqueue);
-	z_erofs_destroy_pcluster_pool();
+	struct kthread_worker *worker;
+	unsigned int cpu;
+
+	for_each_possible_cpu(cpu) {
+		worker = rcu_dereference_protected(
+					z_erofs_pcpu_workers[cpu], 1);
+		rcu_assign_pointer(z_erofs_pcpu_workers[cpu], NULL);
+		if (worker)
+			kthread_destroy_worker(worker);
+	}
+	kfree(z_erofs_pcpu_workers);
 }
 
-static inline int z_erofs_init_workqueue(void)
+static struct kthread_worker *erofs_init_percpu_worker(int cpu)
 {
-	const unsigned int onlinecpus = num_possible_cpus();
+	struct kthread_worker *worker =
+		kthread_create_worker_on_cpu(cpu, 0, "erofs_worker/%u", cpu);
 
-	/*
-	 * 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;
+	if (IS_ERR(worker))
+		return worker;
+	if (IS_ENABLED(CONFIG_EROFS_FS_PCPU_KTHREAD_HIPRI))
+		sched_set_fifo_low(worker->task);
+	else
+		sched_set_normal(worker->task, 0);
+	return worker;
+}
+
+static int erofs_init_percpu_workers(void)
+{
+	struct kthread_worker *worker;
+	unsigned int cpu;
+
+	z_erofs_pcpu_workers = kcalloc(num_possible_cpus(),
+			sizeof(struct kthread_worker *), GFP_ATOMIC);
+	if (!z_erofs_pcpu_workers)
+		return -ENOMEM;
+
+	for_each_online_cpu(cpu) {	/* could miss cpu{off,on}line? */
+		worker = erofs_init_percpu_worker(cpu);
+		if (!IS_ERR(worker))
+			rcu_assign_pointer(z_erofs_pcpu_workers[cpu], worker);
+	}
+	return 0;
+}
+#else
+static inline void erofs_destroy_percpu_workers(void) {}
+static inline int erofs_init_percpu_workers(void) { return 0; }
+#endif
+
+#if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_EROFS_FS_PCPU_KTHREAD)
+static DEFINE_SPINLOCK(z_erofs_pcpu_worker_lock);
+static enum cpuhp_state erofs_cpuhp_state;
+
+static int erofs_cpu_online(unsigned int cpu)
+{
+	struct kthread_worker *worker, *old;
+
+	worker = erofs_init_percpu_worker(cpu);
+	if (IS_ERR(worker))
+		return PTR_ERR(worker);
+
+	spin_lock(&z_erofs_pcpu_worker_lock);
+	old = rcu_dereference_protected(z_erofs_pcpu_workers[cpu],
+			lockdep_is_held(&z_erofs_pcpu_worker_lock));
+	if (!old)
+		rcu_assign_pointer(z_erofs_pcpu_workers[cpu], worker);
+	spin_unlock(&z_erofs_pcpu_worker_lock);
+	if (old)
+		kthread_destroy_worker(worker);
+	return 0;
+}
+
+static int erofs_cpu_offline(unsigned int cpu)
+{
+	struct kthread_worker *worker;
+
+	spin_lock(&z_erofs_pcpu_worker_lock);
+	worker = rcu_dereference_protected(z_erofs_pcpu_workers[cpu],
+			lockdep_is_held(&z_erofs_pcpu_worker_lock));
+	rcu_assign_pointer(z_erofs_pcpu_workers[cpu], NULL);
+	spin_unlock(&z_erofs_pcpu_worker_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 || !CONFIG_EROFS_FS_PCPU_KTHREAD */
+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_percpu_workers();
+	destroy_workqueue(z_erofs_workqueue);
+	z_erofs_destroy_pcluster_pool();
 }
 
 int __init z_erofs_init_zip_subsystem(void)
@@ -366,10 +471,29 @@ 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;
+
+	z_erofs_workqueue = alloc_workqueue("erofs_worker",
+			WQ_UNBOUND | WQ_HIGHPRI, num_possible_cpus());
+	if (!z_erofs_workqueue)
+		goto out_error_workqueue_init;
+
+	err = erofs_init_percpu_workers();
 	if (err)
-		z_erofs_destroy_pcluster_pool();
+		goto out_error_pcpu_worker;
+
+	err = erofs_cpu_hotplug_init();
+	if (err < 0)
+		goto out_error_cpuhp_init;
+	return err;
+
+out_error_cpuhp_init:
+	erofs_destroy_percpu_workers();
+out_error_pcpu_worker:
+	destroy_workqueue(z_erofs_workqueue);
+out_error_workqueue_init:
+	z_erofs_destroy_pcluster_pool();
+out_error_pcluster_pool:
 	return err;
 }
 
@@ -1305,11 +1429,17 @@ static void z_erofs_decompressqueue_work(struct work_struct *work)
 
 	DBG_BUGON(bgq->head == Z_EROFS_PCLUSTER_TAIL_CLOSED);
 	z_erofs_decompress_queue(bgq, &pagepool);
-
 	erofs_release_pages(&pagepool);
 	kvfree(bgq);
 }
 
+#ifdef CONFIG_EROFS_FS_PCPU_KTHREAD
+static void z_erofs_decompressqueue_kthread_work(struct kthread_work *work)
+{
+	z_erofs_decompressqueue_work((struct work_struct *)work);
+}
+#endif
+
 static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
 				       int bios)
 {
@@ -1324,9 +1454,24 @@ 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_)work and sync decompression for atomic contexts only */
 	if (in_atomic() || irqs_disabled()) {
+#ifdef CONFIG_EROFS_FS_PCPU_KTHREAD
+		struct kthread_worker *worker;
+
+		rcu_read_lock();
+		worker = rcu_dereference(
+				z_erofs_pcpu_workers[raw_smp_processor_id()]);
+		if (!worker) {
+			INIT_WORK(&io->u.work, z_erofs_decompressqueue_work);
+			queue_work(z_erofs_workqueue, &io->u.work);
+		} else {
+			kthread_queue_work(worker, &io->u.kthread_work);
+		}
+		rcu_read_unlock();
+#else
 		queue_work(z_erofs_workqueue, &io->u.work);
+#endif
 		/* enable sync decompression for readahead */
 		if (sbi->opt.sync_decompress == EROFS_SYNC_DECOMPRESS_AUTO)
 			sbi->opt.sync_decompress = EROFS_SYNC_DECOMPRESS_FORCE_ON;
@@ -1455,7 +1600,12 @@ static struct z_erofs_decompressqueue *jobqueue_init(struct super_block *sb,
 			*fg = true;
 			goto fg_out;
 		}
+#ifdef CONFIG_EROFS_FS_PCPU_KTHREAD
+		kthread_init_work(&q->u.kthread_work,
+				  z_erofs_decompressqueue_kthread_work);
+#else
 		INIT_WORK(&q->u.work, z_erofs_decompressqueue_work);
+#endif
 	} else {
 fg_out:
 		q = fgq;
@@ -1640,7 +1790,7 @@ 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 decompression but drop it directly instead.
 	 */
 	if (!*force_fg && !nr_bios) {
 		kvfree(q[JQ_SUBMIT]);
-- 
2.24.4