RFC: use a TASK_FIFO kthread for read completion support

Commit 3fffb58 ("erofs: add per-cpu threads for decompression as an
option") explains why workqueue aren't great for low-latency completion
handling.  Switch to a per-cpu kthread to handle it instead.  This code
is based on the erofs code in the above commit, but further simplified
by directly using a kthread instead of a kthread_work.

Signed-off-by: Christoph Hellwig <[email protected]>

Author: Christoph Hellwig

block/bio.c

@@ -19,7 +19,7 @@
 #include <linux/blk-crypto.h>
 #include <linux/xarray.h>
 #include <linux/kmemleak.h>
-#include <linux/llist.h>
+#include <linux/freezer.h>
 
 #include <trace/events/block.h>
 #include "blk.h"
@@ -1719,51 +1719,83 @@ void bio_check_pages_dirty(struct bio *bio)
 EXPORT_SYMBOL_GPL(bio_check_pages_dirty);
 
 struct bio_complete_batch {
-	struct llist_head list;
-	struct delayed_work work;
-	int cpu;
+	spinlock_t lock;
+	struct bio_list bios;
+	struct task_struct *worker;
 };
 
 static DEFINE_PER_CPU(struct bio_complete_batch, bio_complete_batch);
-static struct workqueue_struct *bio_complete_wq;
 
-static void bio_complete_work_fn(struct work_struct *w)
+static bool bio_try_complete_batch(struct bio_complete_batch *batch)
 {
-	struct delayed_work *dw = to_delayed_work(w);
-	struct bio_complete_batch *batch =
-		container_of(dw, struct bio_complete_batch, work);
-	struct llist_node *node;
-	struct bio *bio, *next;
+	struct bio_list bios;
+	unsigned long flags;
+	struct bio *bio;
 
-	do {
-		node = llist_del_all(&batch->list);
-		if (!node)
-			break;
+	spin_lock_irqsave(&batch->lock, flags);
+	bios = batch->bios;
+	bio_list_init(&batch->bios);
+	spin_unlock_irqrestore(&batch->lock, flags);
 
-		node = llist_reverse_order(node);
-		llist_for_each_entry_safe(bio, next, node, bi_llist)
-			bio->bi_end_io(bio);
+	if (bio_list_empty(&bios))
+		return false;
 
-		if (need_resched()) {
-			if (!llist_empty(&batch->list))
-				mod_delayed_work_on(batch->cpu,
-						    bio_complete_wq,
-						    &batch->work, 0);
-			break;
-		}
-	} while (1);
+	__set_current_state(TASK_RUNNING);
+	while ((bio = bio_list_pop(&bios)))
+		bio->bi_end_io(bio);
+	return true;
+}
+
+static int bio_complete_thread(void *private)
+{
+	struct bio_complete_batch *batch = private;
+
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		if (!bio_try_complete_batch(batch))
+			schedule();
+	}
+
+	return 0;
 }
 
 void __bio_complete_in_task(struct bio *bio)
 {
-	struct bio_complete_batch *batch = this_cpu_ptr(&bio_complete_batch);
+	struct bio_complete_batch *batch;
+	unsigned long flags;
+	bool wake;
+
+	get_cpu();
+	batch = this_cpu_ptr(&bio_complete_batch);
+	spin_lock_irqsave(&batch->lock, flags);
+	wake = bio_list_empty(&batch->bios);
+	bio_list_add(&batch->bios, bio);
+	spin_unlock_irqrestore(&batch->lock, flags);
+	put_cpu();
 
-	if (llist_add(&bio->bi_llist, &batch->list))
-		mod_delayed_work_on(batch->cpu, bio_complete_wq,
-				    &batch->work, 1);
+	if (wake)
+		wake_up_process(batch->worker);
 }
 EXPORT_SYMBOL_GPL(__bio_complete_in_task);
 
+static void __init bio_complete_batch_init(int cpu)
+{
+	struct bio_complete_batch *batch =
+		per_cpu_ptr(&bio_complete_batch, cpu);
+	struct task_struct *worker;
+
+	worker = kthread_create_on_cpu(bio_complete_thread,
+			per_cpu_ptr(&bio_complete_batch, cpu),
+			cpu, "bio_worker/%u");
+	if (IS_ERR(worker))
+		panic("bio: can't create kthread_work");
+	sched_set_fifo_low(worker);
+
+	spin_lock_init(&batch->lock);
+	bio_list_init(&batch->bios);
+	batch->worker = worker;
+}
+
 static inline bool bio_remaining_done(struct bio *bio)
 {
 	/*
@@ -2029,16 +2061,7 @@ EXPORT_SYMBOL(bioset_init);
  */
 static int bio_complete_batch_cpu_dead(unsigned int cpu)
 {
-	struct bio_complete_batch *batch =
-		per_cpu_ptr(&bio_complete_batch, cpu);
-	struct llist_node *node;
-	struct bio *bio, *next;
-
-	node = llist_del_all(&batch->list);
-	node = llist_reverse_order(node);
-	llist_for_each_entry_safe(bio, next, node, bi_llist)
-		bio->bi_end_io(bio);
-
+	bio_try_complete_batch(per_cpu_ptr(&bio_complete_batch, cpu));
 	return 0;
 }
 
@@ -2056,18 +2079,8 @@ static int __init init_bio(void)
 				SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
 	}
 
-	for_each_possible_cpu(i) {
-		struct bio_complete_batch *batch =
-			per_cpu_ptr(&bio_complete_batch, i);
-
-		init_llist_head(&batch->list);
-		INIT_DELAYED_WORK(&batch->work, bio_complete_work_fn);
-		batch->cpu = i;
-	}
-
-	bio_complete_wq = alloc_workqueue("bio_complete", WQ_MEM_RECLAIM, 0);
-	if (!bio_complete_wq)
-		panic("bio: can't allocate bio_complete workqueue\n");
+	for_each_possible_cpu(i)
+		bio_complete_batch_init(i);
 
 	cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "block/bio:complete:dead",
 				NULL, bio_complete_batch_cpu_dead);