Re: [PATCH] cfq: get rid of cfqq hash

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Apr 24 2007, Jens Axboe wrote:
> On Tue, Apr 24 2007, Vasily Tarasov wrote:
> > From: Vasily Tarasov <vtaras@openvz.org>
> > 
> > cfq hash is no more necessary.  We always can get cfqq from io context.
> > cfq_get_io_context_noalloc() function is introduced, because we don't want to
> > allocate cic on merging and checking may_queue.
> > In order to identify sync queue we've used hash key = CFQ_KEY_ASYNC. Since hash
> > is eliminated we need to use other criterion: sync flag for queue is added.
> > In all places where we dig in rb_tree we're in current context, so no
> > additional locking is required.
> > 
> > Advantages of this patch: no additional memory for hash, no seeking in hash,
> > code is cleaner. But it is necessary now to seek cic in per-ioc rbtree, but
> > it is faster:
> > - most processes work only with few devices
> > - most systems have only few block devices
> > - it is a rb-tree
> 
> Vasily, this is still not against the CFQ branch, I get tons of rejects:
> 
> axboe@nelson:/src/linux-2.6-block $ patch -p1 --dry-run < ~/foo
> [...]
> 10 out of 27 hunks FAILED -- saving rejects to file
> block/cfq-iosched.c.rej
> 
> If you don't want to use the git tree, then just grab
> 
> http://brick.kernel.dk/snaps/cfq-update-20070424
> 
> and apply it to 2.6.21-rc7-gitX (latest) and provide a diff against
> that. Thanks!

I merged it myself, care to double check it? I'll do some testing on it
tomorrow, and integrate if I'm happy with it.

Have you done any multi disk testing? scsi_debug can be quite handy for
such things, testing thousands of io_contexts and disks. Just be sure to
use delay=1 and fake_rw=1.

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 8093733..e42c09b 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -9,7 +9,6 @@
 #include <linux/module.h>
 #include <linux/blkdev.h>
 #include <linux/elevator.h>
-#include <linux/hash.h>
 #include <linux/rbtree.h>
 #include <linux/ioprio.h>
 
@@ -38,14 +37,6 @@ static int cfq_slice_idle = HZ / 125;
 
 #define CFQ_SLICE_SCALE		(5)
 
-#define CFQ_KEY_ASYNC		(0)
-
-/*
- * for the hash of cfqq inside the cfqd
- */
-#define CFQ_QHASH_SHIFT		6
-#define CFQ_QHASH_ENTRIES	(1 << CFQ_QHASH_SHIFT)
-
 #define RQ_CIC(rq)		((struct cfq_io_context*)(rq)->elevator_private)
 #define RQ_CFQQ(rq)		((rq)->elevator_private2)
 
@@ -62,8 +53,6 @@ static struct completion *ioc_gone;
 #define ASYNC			(0)
 #define SYNC			(1)
 
-#define cfq_cfqq_sync(cfqq)	((cfqq)->key != CFQ_KEY_ASYNC)
-
 #define sample_valid(samples)	((samples) > 80)
 
 /*
@@ -90,11 +79,6 @@ struct cfq_data {
 	struct cfq_rb_root service_tree;
 	unsigned int busy_queues;
 
-	/*
-	 * cfqq lookup hash
-	 */
-	struct hlist_head *cfq_hash;
-
 	int rq_in_driver;
 	int sync_flight;
 	int hw_tag;
@@ -138,10 +122,6 @@ struct cfq_queue {
 	atomic_t ref;
 	/* parent cfq_data */
 	struct cfq_data *cfqd;
-	/* cfqq lookup hash */
-	struct hlist_node cfq_hash;
-	/* hash key */
-	unsigned int key;
 	/* service_tree member */
 	struct rb_node rb_node;
 	/* service_tree key */
@@ -186,6 +166,7 @@ enum cfqq_state_flags {
 	CFQ_CFQQ_FLAG_prio_changed,	/* task priority has changed */
 	CFQ_CFQQ_FLAG_queue_new,	/* queue never been serviced */
 	CFQ_CFQQ_FLAG_slice_new,	/* no requests dispatched in slice */
+	CFQ_CFQQ_FLAG_sync,		/* synchronous queue */
 };
 
 #define CFQ_CFQQ_FNS(name)						\
@@ -212,11 +193,14 @@ CFQ_CFQQ_FNS(idle_window);
 CFQ_CFQQ_FNS(prio_changed);
 CFQ_CFQQ_FNS(queue_new);
 CFQ_CFQQ_FNS(slice_new);
+CFQ_CFQQ_FNS(sync);
 #undef CFQ_CFQQ_FNS
 
-static struct cfq_queue *cfq_find_cfq_hash(struct cfq_data *, unsigned int, unsigned short);
+static struct cfq_io_context * cfq_get_io_context_noalloc(struct cfq_data *,
+							  struct task_struct *);
 static void cfq_dispatch_insert(request_queue_t *, struct request *);
-static struct cfq_queue *cfq_get_queue(struct cfq_data *, unsigned int, struct task_struct *, gfp_t);
+static struct cfq_queue *cfq_get_queue(struct cfq_data *, int,
+				       struct task_struct *, gfp_t);
 
 /*
  * scheduler run of queue, if there are requests pending and no one in the
@@ -235,17 +219,6 @@ static int cfq_queue_empty(request_queue_t *q)
 	return !cfqd->busy_queues;
 }
 
-static inline pid_t cfq_queue_pid(struct task_struct *task, int rw, int is_sync)
-{
-	/*
-	 * Use the per-process queue, for read requests and syncronous writes
-	 */
-	if (!(rw & REQ_RW) || is_sync)
-		return task->pid;
-
-	return CFQ_KEY_ASYNC;
-}
-
 /*
  * Scale schedule slice based on io priority. Use the sync time slice only
  * if a queue is marked sync and has sync io queued. A sync queue with async
@@ -602,10 +575,14 @@ static struct request *
 cfq_find_rq_fmerge(struct cfq_data *cfqd, struct bio *bio)
 {
 	struct task_struct *tsk = current;
-	pid_t key = cfq_queue_pid(tsk, bio_data_dir(bio), bio_sync(bio));
+	struct cfq_io_context *cic;
 	struct cfq_queue *cfqq;
 
-	cfqq = cfq_find_cfq_hash(cfqd, key, tsk->ioprio);
+	cic = cfq_get_io_context_noalloc(cfqd, tsk);
+	if (!cic)
+		return NULL;
+
+	cfqq =  cic->cfqq[bio_sync(bio)];
 	if (cfqq) {
 		sector_t sector = bio->bi_sector + bio_sectors(bio);
 
@@ -699,9 +676,8 @@ static int cfq_allow_merge(request_queue_t *q, struct request *rq,
 			   struct bio *bio)
 {
 	struct cfq_data *cfqd = q->elevator->elevator_data;
-	const int rw = bio_data_dir(bio);
+	struct cfq_io_context *cic;
 	struct cfq_queue *cfqq;
-	pid_t key;
 
 	/*
 	 * Disallow merge of a sync bio into an async request.
@@ -713,9 +689,11 @@ static int cfq_allow_merge(request_queue_t *q, struct request *rq,
 	 * Lookup the cfqq that this bio will be queued with. Allow
 	 * merge only if rq is queued there.
 	 */
-	key = cfq_queue_pid(current, rw, bio_sync(bio));
-	cfqq = cfq_find_cfq_hash(cfqd, key, current->ioprio);
+	cic = cfq_get_io_context_noalloc(cfqd, current);
+	if (!cic)
+		return 0;
 
+	cfqq = cic->cfqq[bio_sync(bio)];
 	if (cfqq == RQ_CFQQ(rq))
 		return 1;
 
@@ -1148,37 +1126,9 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
 		cfq_schedule_dispatch(cfqd);
 	}
 
-	/*
-	 * it's on the empty list and still hashed
-	 */
-	hlist_del(&cfqq->cfq_hash);
 	kmem_cache_free(cfq_pool, cfqq);
 }
 
-static struct cfq_queue *
-__cfq_find_cfq_hash(struct cfq_data *cfqd, unsigned int key, unsigned int prio,
-		    const int hashval)
-{
-	struct hlist_head *hash_list = &cfqd->cfq_hash[hashval];
-	struct hlist_node *entry;
-	struct cfq_queue *__cfqq;
-
-	hlist_for_each_entry(__cfqq, entry, hash_list, cfq_hash) {
-		const unsigned short __p = IOPRIO_PRIO_VALUE(__cfqq->org_ioprio_class, __cfqq->org_ioprio);
-
-		if (__cfqq->key == key && (__p == prio || !prio))
-			return __cfqq;
-	}
-
-	return NULL;
-}
-
-static struct cfq_queue *
-cfq_find_cfq_hash(struct cfq_data *cfqd, unsigned int key, unsigned short prio)
-{
-	return __cfq_find_cfq_hash(cfqd, key, prio, hash_long(key, CFQ_QHASH_SHIFT));
-}
-
 static void cfq_free_io_context(struct io_context *ioc)
 {
 	struct cfq_io_context *__cic;
@@ -1336,7 +1286,7 @@ static inline void changed_ioprio(struct cfq_io_context *cic)
 	cfqq = cic->cfqq[ASYNC];
 	if (cfqq) {
 		struct cfq_queue *new_cfqq;
-		new_cfqq = cfq_get_queue(cfqd, CFQ_KEY_ASYNC, cic->ioc->task,
+		new_cfqq = cfq_get_queue(cfqd, ASYNC, cic->ioc->task,
 					 GFP_ATOMIC);
 		if (new_cfqq) {
 			cic->cfqq[ASYNC] = new_cfqq;
@@ -1368,16 +1318,16 @@ static void cfq_ioc_set_ioprio(struct io_context *ioc)
 }
 
 static struct cfq_queue *
-cfq_get_queue(struct cfq_data *cfqd, unsigned int key, struct task_struct *tsk,
+cfq_get_queue(struct cfq_data *cfqd, int is_sync, struct task_struct *tsk,
 	      gfp_t gfp_mask)
 {
-	const int hashval = hash_long(key, CFQ_QHASH_SHIFT);
 	struct cfq_queue *cfqq, *new_cfqq = NULL;
-	unsigned short ioprio;
+	struct cfq_io_context *cic;
 
 retry:
-	ioprio = tsk->ioprio;
-	cfqq = __cfq_find_cfq_hash(cfqd, key, ioprio, hashval);
+	cic = cfq_get_io_context_noalloc(cfqd, tsk);
+	/* cic always exists here */
+	cfqq = cic->cfqq[is_sync];
 
 	if (!cfqq) {
 		if (new_cfqq) {
@@ -1402,17 +1352,16 @@ retry:
 
 		memset(cfqq, 0, sizeof(*cfqq));
 
-		INIT_HLIST_NODE(&cfqq->cfq_hash);
 		RB_CLEAR_NODE(&cfqq->rb_node);
 		INIT_LIST_HEAD(&cfqq->fifo);
 
-		cfqq->key = key;
-		hlist_add_head(&cfqq->cfq_hash, &cfqd->cfq_hash[hashval]);
 		atomic_set(&cfqq->ref, 0);
 		cfqq->cfqd = cfqd;
 
-		if (key != CFQ_KEY_ASYNC)
+		if (!is_sync)
 			cfq_mark_cfqq_idle_window(cfqq);
+		else
+			cfq_mark_cfqq_sync(cfqq);
 
 		cfq_mark_cfqq_prio_changed(cfqq);
 		cfq_mark_cfqq_queue_new(cfqq);
@@ -1548,6 +1497,19 @@ err:
 	return NULL;
 }
 
+static struct cfq_io_context *
+cfq_get_io_context_noalloc(struct cfq_data *cfqd, struct task_struct *tsk)
+{
+	struct cfq_io_context *cic = NULL;
+	struct io_context *ioc;
+
+	ioc = tsk->io_context;
+	if (ioc)
+		cic = cfq_cic_rb_lookup(cfqd, ioc);
+
+	return cic;
+}
+
 static void
 cfq_update_io_thinktime(struct cfq_data *cfqd, struct cfq_io_context *cic)
 {
@@ -1839,10 +1801,8 @@ static int cfq_may_queue(request_queue_t *q, int rw)
 {
 	struct cfq_data *cfqd = q->elevator->elevator_data;
 	struct task_struct *tsk = current;
+	struct cfq_io_context *cic;
 	struct cfq_queue *cfqq;
-	unsigned int key;
-
-	key = cfq_queue_pid(tsk, rw, rw & REQ_RW_SYNC);
 
 	/*
 	 * don't force setup of a queue from here, as a call to may_queue
@@ -1850,7 +1810,11 @@ static int cfq_may_queue(request_queue_t *q, int rw)
 	 * so just lookup a possibly existing queue, or return 'may queue'
 	 * if that fails
 	 */
-	cfqq = cfq_find_cfq_hash(cfqd, key, tsk->ioprio);
+	cic = cfq_get_io_context_noalloc(cfqd, tsk);
+	if (!cic)
+		return ELV_MQUEUE_MAY; 
+
+	cfqq = cic->cfqq[rw & REQ_RW_SYNC];
 	if (cfqq) {
 		cfq_init_prio_data(cfqq);
 		cfq_prio_boost(cfqq);
@@ -1894,7 +1858,6 @@ cfq_set_request(request_queue_t *q, struct request *rq, gfp_t gfp_mask)
 	struct cfq_io_context *cic;
 	const int rw = rq_data_dir(rq);
 	const int is_sync = rq_is_sync(rq);
-	pid_t key = cfq_queue_pid(tsk, rw, is_sync);
 	struct cfq_queue *cfqq;
 	unsigned long flags;
 
@@ -1908,7 +1871,8 @@ cfq_set_request(request_queue_t *q, struct request *rq, gfp_t gfp_mask)
 		goto queue_fail;
 
 	if (!cic->cfqq[is_sync]) {
-		cfqq = cfq_get_queue(cfqd, key, tsk, gfp_mask);
+		cfqq = cfq_get_queue(cfqd, is_sync, tsk, gfp_mask);
+
 		if (!cfqq)
 			goto queue_fail;
 
@@ -2044,14 +2008,12 @@ static void cfq_exit_queue(elevator_t *e)
 
 	cfq_shutdown_timer_wq(cfqd);
 
-	kfree(cfqd->cfq_hash);
 	kfree(cfqd);
 }
 
 static void *cfq_init_queue(request_queue_t *q)
 {
 	struct cfq_data *cfqd;
-	int i;
 
 	cfqd = kmalloc_node(sizeof(*cfqd), GFP_KERNEL, q->node);
 	if (!cfqd)
@@ -2062,13 +2024,6 @@ static void *cfq_init_queue(request_queue_t *q)
 	cfqd->service_tree = CFQ_RB_ROOT;
 	INIT_LIST_HEAD(&cfqd->cic_list);
 
-	cfqd->cfq_hash = kmalloc_node(sizeof(struct hlist_head) * CFQ_QHASH_ENTRIES, GFP_KERNEL, q->node);
-	if (!cfqd->cfq_hash)
-		goto out_free;
-
-	for (i = 0; i < CFQ_QHASH_ENTRIES; i++)
-		INIT_HLIST_HEAD(&cfqd->cfq_hash[i]);
-
 	cfqd->queue = q;
 
 	init_timer(&cfqd->idle_slice_timer);
@@ -2092,9 +2047,6 @@ static void *cfq_init_queue(request_queue_t *q)
 	cfqd->cfq_slice_idle = cfq_slice_idle;
 
 	return cfqd;
-out_free:
-	kfree(cfqd);
-	return NULL;
 }
 
 static void cfq_slab_kill(void)

-- 
Jens Axboe