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Subject[086/107] cfq: merge cooperating cfq_queues
2.6.32-longterm review patch.  If anyone has any objections, please let us know.

------------------


From: Jeff Moyer <jmoyer@redhat.com>

commit df5fe3e8e13883f58dc97489076bbcc150789a21 upstream.

When cooperating cfq_queues are detected currently, they are allowed to
skip ahead in the scheduling order. It is much more efficient to
automatically share the cfq_queue data structure between cooperating processes.
Performance of the read-test2 benchmark (which is written to emulate the
dump(8) utility) went from 12MB/s to 90MB/s on my SATA disk. NFS servers
with multiple nfsd threads also saw performance increases.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
block/cfq-iosched.c | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 87 insertions(+), 2 deletions(-)

--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -118,6 +118,8 @@ struct cfq_queue {
sector_t last_request_pos;

pid_t pid;
+
+ struct cfq_queue *new_cfqq;
};

/*
@@ -1052,6 +1054,12 @@ static struct cfq_queue *cfq_close_coope
if (!cfqq)
return NULL;

+ /*
+ * It only makes sense to merge sync queues.
+ */
+ if (!cfq_cfqq_sync(cfqq))
+ return NULL;
+
if (cfq_cfqq_coop(cfqq))
return NULL;

@@ -1173,6 +1181,43 @@ cfq_prio_to_maxrq(struct cfq_data *cfqd,
}

/*
+ * Must be called with the queue_lock held.
+ */
+static int cfqq_process_refs(struct cfq_queue *cfqq)
+{
+ int process_refs, io_refs;
+
+ io_refs = cfqq->allocated[READ] + cfqq->allocated[WRITE];
+ process_refs = atomic_read(&cfqq->ref) - io_refs;
+ BUG_ON(process_refs < 0);
+ return process_refs;
+}
+
+static void cfq_setup_merge(struct cfq_queue *cfqq, struct cfq_queue *new_cfqq)
+{
+ int process_refs;
+ struct cfq_queue *__cfqq;
+
+ /* Avoid a circular list and skip interim queue merges */
+ while ((__cfqq = new_cfqq->new_cfqq)) {
+ if (__cfqq == cfqq)
+ return;
+ new_cfqq = __cfqq;
+ }
+
+ process_refs = cfqq_process_refs(cfqq);
+ /*
+ * If the process for the cfqq has gone away, there is no
+ * sense in merging the queues.
+ */
+ if (process_refs == 0)
+ return;
+
+ cfqq->new_cfqq = new_cfqq;
+ atomic_add(process_refs, &new_cfqq->ref);
+}
+
+/*
* Select a queue for service. If we have a current active queue,
* check whether to continue servicing it, or retrieve and set a new one.
*/
@@ -1201,11 +1246,14 @@ static struct cfq_queue *cfq_select_queu
* If another queue has a request waiting within our mean seek
* distance, let it run. The expire code will check for close
* cooperators and put the close queue at the front of the service
- * tree.
+ * tree. If possible, merge the expiring queue with the new cfqq.
*/
new_cfqq = cfq_close_cooperator(cfqd, cfqq, 0);
- if (new_cfqq)
+ if (new_cfqq) {
+ if (!cfqq->new_cfqq)
+ cfq_setup_merge(cfqq, new_cfqq);
goto expire;
+ }

/*
* No requests pending. If the active queue still has requests in
@@ -1516,11 +1564,29 @@ static void cfq_free_io_context(struct i

static void cfq_exit_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
{
+ struct cfq_queue *__cfqq, *next;
+
if (unlikely(cfqq == cfqd->active_queue)) {
__cfq_slice_expired(cfqd, cfqq, 0);
cfq_schedule_dispatch(cfqd);
}

+ /*
+ * If this queue was scheduled to merge with another queue, be
+ * sure to drop the reference taken on that queue (and others in
+ * the merge chain). See cfq_setup_merge and cfq_merge_cfqqs.
+ */
+ __cfqq = cfqq->new_cfqq;
+ while (__cfqq) {
+ if (__cfqq == cfqq) {
+ WARN(1, "cfqq->new_cfqq loop detected\n");
+ break;
+ }
+ next = __cfqq->new_cfqq;
+ cfq_put_queue(__cfqq);
+ __cfqq = next;
+ }
+
cfq_put_queue(cfqq);
}

@@ -2342,6 +2408,16 @@ static void cfq_put_request(struct reque
}
}

+static struct cfq_queue *
+cfq_merge_cfqqs(struct cfq_data *cfqd, struct cfq_io_context *cic,
+ struct cfq_queue *cfqq)
+{
+ cfq_log_cfqq(cfqd, cfqq, "merging with queue %p", cfqq->new_cfqq);
+ cic_set_cfqq(cic, cfqq->new_cfqq, 1);
+ cfq_put_queue(cfqq);
+ return cic_to_cfqq(cic, 1);
+}
+
/*
* Allocate cfq data structures associated with this request.
*/
@@ -2368,6 +2444,15 @@ cfq_set_request(struct request_queue *q,
if (!cfqq || cfqq == &cfqd->oom_cfqq) {
cfqq = cfq_get_queue(cfqd, is_sync, cic->ioc, gfp_mask);
cic_set_cfqq(cic, cfqq, is_sync);
+ } else {
+ /*
+ * Check to see if this queue is scheduled to merge with
+ * another, closely cooperating queue. The merging of
+ * queues happens here as it must be done in process context.
+ * The reference on new_cfqq was taken in merge_cfqqs.
+ */
+ if (cfqq->new_cfqq)
+ cfqq = cfq_merge_cfqqs(cfqd, cic, cfqq);
}

cfqq->allocated[rw]++;



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