Re: dd hangs when reading large partitions

[Jens Axboe <axboe@kernel.dk>]

On 1/18/19 8:18 AM, jianchao.wang wrote:
> Hello
> 
> On 1/18/19 8:10 PM, Marc Gonzalez wrote:
>> Hello,
>>
>> I'm running into an issue which I don't know how to debug.
>> So I'm open to ideas and suggestions :-)
>>
>> On my arm64 board, I have enabled Universal Flash Storage support.
>>
>> I wanted to benchmark read performance, and noticed that the system
>> locks up when I read partitions larger than 3.5 GB, unless I tell
>> dd to use direct IO:
>>
>> *** WITH O_DIRECT ***
>> # dd if=/dev/sda of=/dev/null bs=1M iflag=direct status=progress
>> 57892929536 bytes (58 GB, 54 GiB) copied, 697.006 s, 83.1 MB/s 
>> 55256+0 records in
>> 55256+0 records out
>> 57940115456 bytes (58 GB, 54 GiB) copied, 697.575 s, 83.1 MB/s
>>
>> *** WITHOUT O_DIRECT ***
>> # dd if=/dev/sda of=/dev/null bs=1M status=progress
>> 3853516800 bytes (3.9 GB, 3.6 GiB) copied, 49.0002 s, 78.6 MB/s
>>
>>
>> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
>> rcu:    1-...0: (8242 ticks this GP) idle=106/1/0x4000000000000000 softirq=168/171 fqs=2626 
>> rcu:    6-...0: (99 GPs behind) idle=ec2/1/0x4000000000000000 softirq=71/71 fqs=2626 
>> rcu:    (detected by 7, t=5254 jiffies, g=-275, q=2)
>> Task dump for CPU 1:
>> kworker/1:1H    R  running task        0   675      2 0x0000002a
>> Workqueue: kblockd blk_mq_run_work_fn
>> Call trace:
>>  __switch_to+0x168/0x1d0
> 
> It looks like the blk_mq_run_work_fn went to sleep with rcu lock (preempt), isn't it ?
> Can you share the symbol of the following address ?

It's UFS that totally buggy, if you look at its queuecommand, it does:

        if (!down_read_trylock(&hba->clk_scaling_lock))                         
                return SCSI_MLQUEUE_HOST_BUSY;

UFS either needs to get fixed up, or we'll want a way to do something like
the below.

Marc, can you test this?


diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index eaf329db3973..e28c3420a9d9 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -412,6 +412,7 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
 	shost->hostt = sht;
 	shost->this_id = sht->this_id;
 	shost->can_queue = sht->can_queue;
+	shost->queue_may_block = sht->queue_may_block;
 	shost->sg_tablesize = sht->sg_tablesize;
 	shost->sg_prot_tablesize = sht->sg_prot_tablesize;
 	shost->cmd_per_lun = sht->cmd_per_lun;
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index b13cc9288ba0..4e266af2871f 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1902,6 +1902,8 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 	shost->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
 	shost->tag_set.flags |=
 		BLK_ALLOC_POLICY_TO_MQ_FLAG(shost->hostt->tag_alloc_policy);
+	if (shost->queue_may_blocK)
+		shost->tag_set.flags |= BLK_MQ_F_BLOCKING;
 	shost->tag_set.driver_data = shost;
 
 	return blk_mq_alloc_tag_set(&shost->tag_set);
diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index 9ba7671b84f8..9ab354e43630 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -6981,6 +6981,7 @@ static struct scsi_host_template ufshcd_driver_template = {
 	.sg_tablesize		= SG_ALL,
 	.cmd_per_lun		= UFSHCD_CMD_PER_LUN,
 	.can_queue		= UFSHCD_CAN_QUEUE,
+	.queue_may_block	= 1,
 	.max_host_blocked	= 1,
 	.track_queue_depth	= 1,
 	.sdev_groups		= ufshcd_driver_groups,
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index 6ca954e9f752..30aa7b6c4342 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -339,6 +339,11 @@ struct scsi_host_template {
 	 */
 	int can_queue;
 
+	/*
+	 * If the ->queuecommand() ever blocks, this should be set
+	 */
+	int queue_may_block;
+
 	/*
 	 * In many instances, especially where disconnect / reconnect are
 	 * supported, our host also has an ID on the SCSI bus.  If this is
@@ -584,6 +589,7 @@ struct Scsi_Host {
 
 	int this_id;
 	int can_queue;
+	int queue_may_block;
 	short cmd_per_lun;
 	short unsigned int sg_tablesize;
 	short unsigned int sg_prot_tablesize;

-- 
Jens Axboe