----- Forwarded message from "Michael S. Tsirkin" <mst@xxxxxxxxxx> -----
From: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Date: Fri, 9 Jun 2023 03:27:28 -0400
To: linux-kernel@xxxxxxxxxxxxxxx
Cc: kernel test robot <lkp@xxxxxxxxx>, Suwan Kim <suwan.kim027@xxxxxxxxx>, "Roberts, Martin" <martin.roberts@xxxxxxxxx>, Jason Wang
<jasowang@xxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Stefan Hajnoczi <stefanha@xxxxxxxxxx>, Xuan Zhuo
<xuanzhuo@xxxxxxxxxxxxxxxxx>, Jens Axboe <axboe@xxxxxxxxx>, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx,
linux-block@xxxxxxxxxxxxxxx
Subject: [PATCH v2] Revert "virtio-blk: support completion batching for the IRQ path"
Message-ID: <336455b4f630f329380a8f53ee8cad3868764d5c.1686295549.git.mst@xxxxxxxxxx>
This reverts commit 07b679f70d73483930e8d3c293942416d9cd5c13.
This change appears to have broken things...
We now see applications hanging during disk accesses.
e.g.
multi-port virtio-blk device running in h/w (FPGA)
Host running a simple 'fio' test.
[global]
thread=1
direct=1
ioengine=libaio
norandommap=1
group_reporting=1
bs=4K
rw=read
iodepth=128
runtime=1
numjobs=4
time_based
[job0]
filename=/dev/vda
[job1]
filename=/dev/vdb
[job2]
filename=/dev/vdc
...
[job15]
filename=/dev/vdp
i.e. 16 disks; 4 queues per disk; simple burst of 4KB reads
This is repeatedly run in a loop.
After a few, normally <10 seconds, fio hangs.
With 64 queues (16 disks), failure occurs within a few seconds; with 8 queues (2 disks) it may take ~hour before hanging.
Last message:
fio-3.19
Starting 8 threads
Jobs: 1 (f=1): [_(7),R(1)][68.3%][eta 03h:11m:06s]
I think this means at the end of the run 1 queue was left incomplete.
'diskstats' (run while fio is hung) shows no outstanding transactions.
e.g.
$ cat /proc/diskstats
...
252 0 vda 1843140071 0 14745120568 712568645 0 0 0 0 0 3117947 712568645 0 0 0 0 0 0
252 16 vdb 1816291511 0 14530332088 704905623 0 0 0 0 0 3117711 704905623 0 0 0 0 0 0
...
Other stats (in the h/w, and added to the virtio-blk driver ([a]virtio_queue_rq(), [b]virtblk_handle_req(), [c]virtblk_request_done()) all agree, and show every request had a completion, and that virtblk_request_done() never gets called.
e.g.
PF= 0 vq=0 1 2 3
[a]request_count - 839416590 813148916 105586179 84988123
[b]completion1_count - 839416590 813148916 105586179 84988123
[c]completion2_count - 0 0 0 0
PF= 1 vq=0 1 2 3
[a]request_count - 823335887 812516140 104582672 75856549
[b]completion1_count - 823335887 812516140 104582672 75856549
[c]completion2_count - 0 0 0 0
i.e. the issue is after the virtio-blk driver.
This change was introduced in kernel 6.3.0.
I am seeing this using 6.3.3.
If I run with an earlier kernel (5.15), it does not occur.
If I make a simple patch to the 6.3.3 virtio-blk driver, to skip the blk_mq_add_to_batch()call, it does not fail.
e.g.
kernel 5.15 - this is OK
virtio_blk.c,virtblk_done() [irq handler]
if (likely(!blk_should_fake_timeout(req->q))) {
blk_mq_complete_request(req);
}
kernel 6.3.3 - this fails
virtio_blk.c,virtblk_handle_req() [irq handler]
if (likely(!blk_should_fake_timeout(req->q))) {
if (!blk_mq_complete_request_remote(req)) {
if (!blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr), virtblk_complete_batch)) {
virtblk_request_done(req); //this never gets called... so blk_mq_add_to_batch() must always succeed
}
}
}
If I do, kernel 6.3.3 - this is OK
virtio_blk.c,virtblk_handle_req() [irq handler]
if (likely(!blk_should_fake_timeout(req->q))) {
if (!blk_mq_complete_request_remote(req)) {
virtblk_request_done(req); //force this here...
if (!blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr), virtblk_complete_batch)) {
virtblk_request_done(req); //this never gets called... so blk_mq_add_to_batch() must always succeed
}
}
}
Perhaps you might like to fix/test/revert this change...
Martin
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202306090826.C1fZmdMe-lkp@xxxxxxxxx/
Cc: Suwan Kim <suwan.kim027@xxxxxxxxx>
Reported-by: "Roberts, Martin" <martin.roberts@xxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
Since v1:
fix build error
Still completely untested as I'm traveling.
Martin, Suwan, could you please test and report?
Suwan if you have a better revert in mind pls post and
I will be happy to drop this.
Thanks!
drivers/block/virtio_blk.c | 82 +++++++++++++++++---------------------
1 file changed, 37 insertions(+), 45 deletions(-)
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 2b918e28acaa..b47358da92a2 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -348,63 +348,33 @@ static inline void virtblk_request_done(struct request *req)
blk_mq_end_request(req, status);
}
-static void virtblk_complete_batch(struct io_comp_batch *iob)
-{
- struct request *req;
-
- rq_list_for_each(&iob->req_list, req) {
- virtblk_unmap_data(req, blk_mq_rq_to_pdu(req));
- virtblk_cleanup_cmd(req);
- }
- blk_mq_end_request_batch(iob);
-}
-
-static int virtblk_handle_req(struct virtio_blk_vq *vq,
- struct io_comp_batch *iob)
-{
- struct virtblk_req *vbr;
- int req_done = 0;
- unsigned int len;
-
- while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
- struct request *req = blk_mq_rq_from_pdu(vbr);
-
- if (likely(!blk_should_fake_timeout(req->q)) &&
- !blk_mq_complete_request_remote(req) &&
- !blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr),
- virtblk_complete_batch))
- virtblk_request_done(req);
- req_done++;
- }
-
- return req_done;
-}
-
static void virtblk_done(struct virtqueue *vq)
{
struct virtio_blk *vblk = vq->vdev->priv;
- struct virtio_blk_vq *vblk_vq = &vblk->vqs[vq->index];
- int req_done = 0;
+ bool req_done = false;
+ int qid = vq->index;
+ struct virtblk_req *vbr;
unsigned long flags;
- DEFINE_IO_COMP_BATCH(iob);
+ unsigned int len;
- spin_lock_irqsave(&vblk_vq->lock, flags);
+ spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
do {
virtqueue_disable_cb(vq);
- req_done += virtblk_handle_req(vblk_vq, &iob);
+ while ((vbr = virtqueue_get_buf(vblk->vqs[qid].vq, &len)) != NULL) {
+ struct request *req = blk_mq_rq_from_pdu(vbr);
+ if (likely(!blk_should_fake_timeout(req->q)))
+ blk_mq_complete_request(req);
+ req_done = true;
+ }
if (unlikely(virtqueue_is_broken(vq)))
break;
} while (!virtqueue_enable_cb(vq));
- if (req_done) {
- if (!rq_list_empty(iob.req_list))
- iob.complete(&iob);
-
- /* In case queue is stopped waiting for more buffers. */
+ /* In case queue is stopped waiting for more buffers. */
+ if (req_done)
blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
- }
- spin_unlock_irqrestore(&vblk_vq->lock, flags);
+ spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
}
static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
@@ -1283,15 +1253,37 @@ static void virtblk_map_queues(struct blk_mq_tag_set *set)
}
}
+static void virtblk_complete_batch(struct io_comp_batch *iob)
+{
+ struct request *req;
+
+ rq_list_for_each(&iob->req_list, req) {
+ virtblk_unmap_data(req, blk_mq_rq_to_pdu(req));
+ virtblk_cleanup_cmd(req);
+ }
+ blk_mq_end_request_batch(iob);
+}
+
static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
{
struct virtio_blk *vblk = hctx->queue->queuedata;
struct virtio_blk_vq *vq = get_virtio_blk_vq(hctx);
+ struct virtblk_req *vbr;
unsigned long flags;
+ unsigned int len;
int found = 0;
spin_lock_irqsave(&vq->lock, flags);
- found = virtblk_handle_req(vq, iob);
+
+ while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
+ struct request *req = blk_mq_rq_from_pdu(vbr);
+
+ found++;
+ if (!blk_mq_complete_request_remote(req) &&
+ !blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr),
+ virtblk_complete_batch))
+ virtblk_request_done(req);
+ }
if (found)
blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
--
MST
----- End forwarded message -----
