Re: [PATCH] blk-mq: Wait for for hctx requests on CPU unplug

[Ming Lei]

On Fri, Apr 05, 2019 at 05:36:32PM -0600, Keith Busch wrote:
> On Fri, Apr 5, 2019 at 5:04 PM Jens Axboe <ax...@kernel.dk> wrote:
> > Looking at current peak testing, I've got around 1.2% in queue enter
> > and exit. It's definitely not free, hence my question. Probably safe
> > to assume that we'll double that cycle counter, per IO.
> 
> Okay, that's not negligible at all. I don't know of a faster reference
> than the percpu_ref, but that much overhead would have to rule out
> having a per hctx counter.

Or not using any refcount in fast path, how about the following one?

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3ff3d7b49969..6fe334e12236 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2199,6 +2199,23 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct 
blk_mq_tags *tags,
        return -ENOMEM;
 }
 
+static void blk_mq_wait_hctx_become_idle(struct blk_mq_hw_ctx *hctx,
+               int dead_cpu)
+{
+       unsigned long msecs_left = 1000 * 10;
+
+       while (msecs_left > 0) {
+               if (blk_mq_hctx_idle(hctx))
+                       break;
+               msleep(5);
+               msecs_left -= 5;
+       }
+
+       if (msecs_left > 0)
+               printk(KERN_WARNING "requests not completed from "
+                       "CPU %d\n", dead_cpu);
+}
+
 /*
  * 'cpu' is going away. splice any existing rq_list entries from this
  * software queue to the hw queue dispatch list, and ensure that it
@@ -2230,6 +2247,14 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, 
struct hlist_node *node)
        spin_unlock(&hctx->lock);
 
        blk_mq_run_hw_queue(hctx, true);
+
+       /*
+        * Interrupt for this queue will be shutdown, so wait until all
+        * requests from this hctx is done or timeout.
+        */
+       if (cpumask_first_and(hctx->cpumask, cpu_online_mask) >= nr_cpu_ids)
+               blk_mq_wait_hctx_become_idle(hctx, cpu);
+
        return 0;
 }
 
diff --git a/block/blk-mq.h b/block/blk-mq.h
index d704fc7766f4..935cf8519bf2 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -240,4 +240,15 @@ static inline void blk_mq_clear_mq_map(struct 
blk_mq_queue_map *qmap)
                qmap->mq_map[cpu] = 0;
 }
 
+static inline bool blk_mq_hctx_idle(struct blk_mq_hw_ctx *hctx)
+{
+       struct blk_mq_tags *tags = hctx->sched_tags ?: hctx->tags;
+
+       if (!tags)
+               return true;
+
+       return !sbitmap_any_bit_set(&tags->bitmap_tags.sb) &&
+                       !sbitmap_any_bit_set(&tags->bitmap_tags.sb);
+}
+
 #endif

Thanks,
Ming