Re: [PATCH 4/5] blk-mq-debugfs: Show busy requests

[Jens Axboe <axboe@kernel.dk>]

On 05/26/2017 08:38 AM, Hannes Reinecke wrote:
> On 05/26/2017 01:38 AM, Bart Van Assche wrote:
>> Requests that got stuck in a block driver are neither on
>> blk_mq_ctx.rq_list nor on any hw dispatch queue. Make these
>> visible in debugfs through the "busy" attribute.
>>
>> Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Hannes Reinecke <hare@suse.com>
>> Cc: Omar Sandoval <osandov@fb.com>
>> Cc: Ming Lei <ming.lei@redhat.com>
>> ---
>>  block/blk-mq-debugfs.c | 25 +++++++++++++++++++++++++
>>  1 file changed, 25 insertions(+)
>>
>> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
>> index 8b06a12c1461..70a2b955afee 100644
>> --- a/block/blk-mq-debugfs.c
>> +++ b/block/blk-mq-debugfs.c
>> @@ -370,6 +370,30 @@ static const struct seq_operations hctx_dispatch_seq_ops = {
>>  	.show	= blk_mq_debugfs_rq_show,
>>  };
>>  
>> +struct show_busy_ctx {
>> +	struct seq_file		*m;
>> +	struct blk_mq_hw_ctx	*hctx;
>> +};
>> +
>> +static void hctx_show_busy(struct request *rq, void *data, bool reserved)
>> +{
>> +	const struct show_busy_ctx *ctx = data;
>> +
>> +	if (blk_mq_map_queue(rq->q, rq->mq_ctx->cpu) == ctx->hctx &&
>> +	    test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
>> +		blk_mq_debugfs_rq_show(ctx->m, &rq->queuelist);
>> +}
>> +
>> +static int hctx_busy_show(void *data, struct seq_file *m)
>> +{
>> +	struct blk_mq_hw_ctx *hctx = data;
>> +	struct show_busy_ctx ctx = { .m = m, .hctx = hctx };
>> +
>> +	blk_mq_tagset_busy_iter(hctx->queue->tag_set, hctx_show_busy, &ctx);
>> +
>> +	return 0;
>> +}
>> +
>>  static int hctx_ctx_map_show(void *data, struct seq_file *m)
>>  {
>>  	struct blk_mq_hw_ctx *hctx = data;
>> @@ -705,6 +729,7 @@ static const struct blk_mq_debugfs_attr blk_mq_debugfs_hctx_attrs[] = {
>>  	{"state", 0400, hctx_state_show},
>>  	{"flags", 0400, hctx_flags_show},
>>  	{"dispatch", 0400, .seq_ops = &hctx_dispatch_seq_ops},
>> +	{"busy", 0400, hctx_busy_show},
>>  	{"ctx_map", 0400, hctx_ctx_map_show},
>>  	{"tags", 0400, hctx_tags_show},
>>  	{"tags_bitmap", 0400, hctx_tags_bitmap_show},
>>
> Hmm. I wonder if this is going to work as intended; 'busy' might be
> changing rapidly, so the busy_iter might be giving us unintended results.
> Have you checked?

That's true for _any_ of the debugfs exports for probing into internals
that hold/store requests. All of them are just a snapshot in time,
there's no intent (or posibility) for these to be stable in any way.

-- 
Jens Axboe