On Fri, Apr 24, 2020 at 03:27:45PM +0200, Hannes Reinecke wrote:
> On 4/24/20 12:23 PM, Ming Lei wrote:
> > Before one CPU becomes offline, check if it is the last online CPU of hctx.
> > If yes, mark this hctx as inactive, meantime wait for completion of all
> > in-flight IOs originated from this hctx. Meantime check if this hctx has
> > become inactive in blk_mq_get_driver_tag(), if yes, release the
> > allocated tag.
> >
> > This way guarantees that there isn't any inflight IO before shutdowning
> > the managed IRQ line when all CPUs of this IRQ line is offline.
> >
> > Cc: John Garry <john.garry@xxxxxxxxxx>
> > Cc: Bart Van Assche <bvanassche@xxxxxxx>
> > Cc: Hannes Reinecke <hare@xxxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> > ---
> > block/blk-mq-debugfs.c | 1 +
> > block/blk-mq.c | 124 +++++++++++++++++++++++++++++++++++++----
> > include/linux/blk-mq.h | 3 +
> > 3 files changed, 117 insertions(+), 11 deletions(-)
> >
> > diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> > index 8e745826eb86..b62390918ca5 100644
> > --- a/block/blk-mq-debugfs.c
> > +++ b/block/blk-mq-debugfs.c
> > @@ -213,6 +213,7 @@ static const char *const hctx_state_name[] = {
> > HCTX_STATE_NAME(STOPPED),
> > HCTX_STATE_NAME(TAG_ACTIVE),
> > HCTX_STATE_NAME(SCHED_RESTART),
> > + HCTX_STATE_NAME(INACTIVE),
> > };
> > #undef HCTX_STATE_NAME
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index d432cc74ef78..4d0c271d9f6f 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -1050,11 +1050,31 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
> > return true;
> > }
> > -static bool blk_mq_get_driver_tag(struct request *rq)
> > +static bool blk_mq_get_driver_tag(struct request *rq, bool direct_issue)
> > {
> > if (rq->tag != -1)
> > return true;
> > - return __blk_mq_get_driver_tag(rq);
> > +
> > + if (!__blk_mq_get_driver_tag(rq))
> > + return false;
> > + /*
> > + * Add one memory barrier in case that direct issue IO process is
> > + * migrated to other CPU which may not belong to this hctx, so we can
> > + * order driver tag assignment and checking BLK_MQ_S_INACTIVE.
> > + * Otherwise, barrier() is enough given both setting BLK_MQ_S_INACTIVE
> > + * and driver tag assignment are run on the same CPU in case that
> > + * BLK_MQ_S_INACTIVE is set.
> > + */
> > + if (unlikely(direct_issue && rq->mq_ctx->cpu != raw_smp_processor_id()))
> > + smp_mb();
> > + else
> > + barrier();
> > +
> > + if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &rq->mq_hctx->state))) {
> > + blk_mq_put_driver_tag(rq);
> > + return false;
> > + }
> > + return true;
> > }
> > static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,
> > @@ -1103,7 +1123,7 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx *hctx,
> > * Don't clear RESTART here, someone else could have set it.
> > * At most this will cost an extra queue run.
> > */
> > - return blk_mq_get_driver_tag(rq);
> > + return blk_mq_get_driver_tag(rq, false);
> > }
> > wait = &hctx->dispatch_wait;
> > @@ -1129,7 +1149,7 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx *hctx,
> > * allocation failure and adding the hardware queue to the wait
> > * queue.
> > */
> > - ret = blk_mq_get_driver_tag(rq);
> > + ret = blk_mq_get_driver_tag(rq, false);
> > if (!ret) {
> > spin_unlock(&hctx->dispatch_wait_lock);
> > spin_unlock_irq(&wq->lock);
> > @@ -1228,7 +1248,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> > break;
> > }
> > - if (!blk_mq_get_driver_tag(rq)) {
> > + if (!blk_mq_get_driver_tag(rq, false)) {
> > /*
> > * The initial allocation attempt failed, so we need to
> > * rerun the hardware queue when a tag is freed. The
> > @@ -1260,7 +1280,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> > bd.last = true;
> > else {
> > nxt = list_first_entry(list, struct request, queuelist);
> > - bd.last = !blk_mq_get_driver_tag(nxt);
> > + bd.last = !blk_mq_get_driver_tag(nxt, false);
> > }
> > ret = q->mq_ops->queue_rq(hctx, &bd);
> > @@ -1853,7 +1873,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> > if (!blk_mq_get_dispatch_budget(hctx))
> > goto insert;
> > - if (!blk_mq_get_driver_tag(rq)) {
> > + if (!blk_mq_get_driver_tag(rq, true)) {
> > blk_mq_put_dispatch_budget(hctx);
> > goto insert;
> > }
> > @@ -2261,13 +2281,92 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
> > return -ENOMEM;
> > }
> > -static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
> > +struct count_inflight_data {
> > + unsigned count;
> > + struct blk_mq_hw_ctx *hctx;
> > +};
> > +
> > +static bool blk_mq_count_inflight_rq(struct request *rq, void *data,
> > + bool reserved)
> > {
> > - return 0;
> > + struct count_inflight_data *count_data = data;
> > +
> > + /*
> > + * Can't check rq's state because it is updated to MQ_RQ_IN_FLIGHT
> > + * in blk_mq_start_request(), at that time we can't prevent this rq
> > + * from being issued.
> > + *
> > + * So check if driver tag is assigned, if yes, count this rq as
> > + * inflight.
> > + */
> > + if (rq->tag >= 0 && rq->mq_hctx == count_data->hctx)
> > + count_data->count++;
> > +
> > + return true;
> > +}
> > +
> > +static bool blk_mq_inflight_rq(struct request *rq, void *data,
> > + bool reserved)
> > +{
> > + return rq->tag >= 0;
> > +}
> > +
> > +static unsigned blk_mq_tags_inflight_rqs(struct blk_mq_hw_ctx *hctx)
> > +{
> > + struct count_inflight_data count_data = {
> > + .count = 0,
> > + .hctx = hctx,
> > + };
> > +
> > + blk_mq_all_tag_busy_iter(hctx->tags, blk_mq_count_inflight_rq,
> > + blk_mq_inflight_rq, &count_data);
> > +
> > + return count_data.count;
> > +}
> > +
>
> Remind me again: Why do we need the 'filter' function here?
> Can't we just move the filter function into the main iterator and
> stay with the original implementation?
This is a good question, cause it takes John and me days for figuring
out reason of one IO hang.
The main iterator only iterates requests which blk_mq_request_started()
returns true, see bt_tags_iter().
However, we prevent rq from being allocated a driver tag in
blk_mq_get_driver_tag(). There is still some distance between assigning
driver tag and calling into blk_mq_start_request() which is called from
.queue_rq().
More importantly we can check nothing in blk_mq_start_request().
Thanks,
Ming
