[PATCH RFC v5 1/3] block: add BIO_COMPLETE_IN_TASK for task-context completion

[Tal Zussman]

Some bio completion handlers need to run in task context but bio_endio()
can be called from IRQ context (e.g. buffer_head writeback). Add a
BIO_COMPLETE_IN_TASK flag that bio submitters can set to request
task-context completion of their bi_end_io callback.

When bio_endio() sees this flag and is running in non-task context, it
queues the bio to a per-cpu lockless list and schedules a delayed work
item to call bi_end_io() from task context.  The delayed work uses a
1-jiffie delay to allow batches of completions to accumulate before
processing. A CPU hotplug dead callback drains any remaining bios from
the departing CPU's batch.

This will be used to enable RWF_DONTCACHE for block devices, and could
be used for other subsystems like fscrypt that need task-context bio
completion.

Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
---
 block/bio.c               | 83 ++++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/blk_types.h |  7 +++-
 2 files changed, 88 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 8203bb7455a9..21b403eb1c04 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -18,6 +18,7 @@
 #include <linux/highmem.h>
 #include <linux/blk-crypto.h>
 #include <linux/xarray.h>
+#include <linux/llist.h>
 
 #include <trace/events/block.h>
 #include "blk.h"
@@ -1714,6 +1715,51 @@ 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;
+};
+
+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)
+{
+	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;
+
+	do {
+		node = llist_del_all(&batch->list);
+		if (!node)
+			break;
+
+		node = llist_reverse_order(node);
+		llist_for_each_entry_safe(bio, next, node, bi_llist)
+			bio->bi_end_io(bio);
+
+		if (need_resched()) {
+			if (!llist_empty(&batch->list))
+				mod_delayed_work_on(batch->cpu,
+						    bio_complete_wq,
+						    &batch->work, 0);
+			break;
+		}
+	} while (1);
+}
+
+static void bio_queue_completion(struct bio *bio)
+{
+	struct bio_complete_batch *batch = this_cpu_ptr(&bio_complete_batch);
+
+	if (llist_add(&bio->bi_llist, &batch->list))
+		mod_delayed_work_on(batch->cpu, bio_complete_wq,
+				    &batch->work, 1);
+}
+
 static inline bool bio_remaining_done(struct bio *bio)
 {
 	/*
@@ -1788,7 +1834,9 @@ void bio_endio(struct bio *bio)
 	}
 #endif
 
-	if (bio->bi_end_io)
+	if (!in_task() && bio_flagged(bio, BIO_COMPLETE_IN_TASK))
+		bio_queue_completion(bio);
+	else if (bio->bi_end_io)
 		bio->bi_end_io(bio);
 }
 EXPORT_SYMBOL(bio_endio);
@@ -1974,6 +2022,24 @@ int bioset_init(struct bio_set *bs,
 }
 EXPORT_SYMBOL(bioset_init);
 
+/*
+ * Drain a dead CPU's deferred bio completions.
+ */
+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);
+
+	return 0;
+}
+
 static int __init init_bio(void)
 {
 	int i;
@@ -1988,6 +2054,21 @@ 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");
+
+	cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "block/bio:complete:dead",
+				NULL, bio_complete_batch_cpu_dead);
 	cpuhp_setup_state_multi(CPUHP_BIO_DEAD, "block/bio:dead", NULL,
 					bio_cpu_dead);
 
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 8808ee76e73c..0b55159d110d 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -11,6 +11,7 @@
 #include <linux/device.h>
 #include <linux/ktime.h>
 #include <linux/rw_hint.h>
+#include <linux/llist.h>
 
 struct bio_set;
 struct bio;
@@ -208,7 +209,10 @@ typedef unsigned int blk_qc_t;
  * stacking drivers)
  */
 struct bio {
-	struct bio		*bi_next;	/* request queue link */
+	union {
+		struct bio	*bi_next;	/* request queue link */
+		struct llist_node bi_llist;	/* deferred completion */
+	};
 	struct block_device	*bi_bdev;
 	blk_opf_t		bi_opf;		/* bottom bits REQ_OP, top bits
 						 * req_flags.
@@ -322,6 +326,7 @@ enum {
 	BIO_REMAPPED,
 	BIO_ZONE_WRITE_PLUGGING, /* bio handled through zone write plugging */
 	BIO_EMULATES_ZONE_APPEND, /* bio emulates a zone append operation */
+	BIO_COMPLETE_IN_TASK, /* complete bi_end_io() in task context */
 	BIO_FLAG_LAST
 };
 

-- 
2.39.5