Re: for-next hangs on test srp/012

[Jens Axboe <axboe@kernel.dk>]

On 12/11/18 6:37 PM, Ming Lei wrote:
> On Tue, Dec 11, 2018 at 06:23:31PM -0700, Jens Axboe wrote:
>> On 12/11/18 6:05 PM, Jens Axboe wrote:
>>> On 12/11/18 5:38 PM, Ming Lei wrote:
>>>> On Wed, Dec 12, 2018 at 8:28 AM Mike Snitzer <snitzer@redhat.com> wrote:
>>>>>
>>>>> On Tue, Dec 11 2018 at  7:19pm -0500,
>>>>> Ming Lei <tom.leiming@gmail.com> wrote:
>>>>>
>>>>>> On Wed, Dec 12, 2018 at 8:04 AM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>>
>>>>>>> On 12/11/18 3:58 PM, Bart Van Assche wrote:
>>>>>>>> Hi Jens,
>>>>>>>>
>>>>>>>> If I run the following subset of blktests:
>>>>>>>>
>>>>>>>>   while :; do ./check -q srp && ./check -q nvmeof-mp; done
>>>>>>>>
>>>>>>>> against today's for-next branch (commit dd2bf2df85a7) then after some
>>>>>>>> time the following hang is reported:
>>>>>>>>
>>>>>>>> INFO: task fio:14869 blocked for more than 120 seconds.
>>>>>>>>       Not tainted 4.20.0-rc6-dbg+ #1
>>>>>>>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>>>>>>> fio             D25272 14869  14195 0x00000000
>>>>>>>> Call Trace:
>>>>>>>>  __schedule+0x401/0xe50
>>>>>>>>  schedule+0x4e/0xd0
>>>>>>>>  io_schedule+0x21/0x50
>>>>>>>>  blk_mq_get_tag+0x46d/0x640
>>>>>>>>  blk_mq_get_request+0x7c0/0xa00
>>>>>>>>  blk_mq_make_request+0x241/0xa70
>>>>>>>>  generic_make_request+0x411/0x950
>>>>>>>>  submit_bio+0x9b/0x250
>>>>>>>>  blkdev_direct_IO+0x7fb/0x870
>>>>>>>>  generic_file_direct_write+0x119/0x210
>>>>>>>>  __generic_file_write_iter+0x11c/0x280
>>>>>>>>  blkdev_write_iter+0x13c/0x220
>>>>>>>>  aio_write+0x204/0x310
>>>>>>>>  io_submit_one+0x9c6/0xe70
>>>>>>>>  __x64_sys_io_submit+0x115/0x340
>>>>>>>>  do_syscall_64+0x71/0x210
>>>>>>>>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>>>>>>
>>>>>>>> When that hang occurs my list-pending-block-requests script does not show
>>>>>>>> any pending requests:
>>>>>>>>
>>>>>>>> # list-pending-block-requests
>>>>>>>> dm-0
>>>>>>>> loop0
>>>>>>>> loop1
>>>>>>>> loop2
>>>>>>>> loop3
>>>>>>>> loop4
>>>>>>>> loop5
>>>>>>>> loop6
>>>>>>>> loop7
>>>>>>>> nullb0
>>>>>>>> nullb1
>>>>>>>> sda
>>>>>>>> sdb
>>>>>>>> sdc
>>>>>>>> sdd
>>>>>>>> vda
>>>>>>>> vdb
>>>>>>>>
>>>>>>>> Enabling fail_if_no_path mode did not resolve the hang so I don't think
>>>>>>>> that the root cause is in any of the dm drivers used in this test:
>>>>>>>>
>>>>>>>> # dmsetup ls | while read dm rest; do dmsetup message $dm 0 fail_if_no_path; done; dmsetup remove_all; dmsetup table
>>>>>>>> 360014056e756c6c62300000000000000: 0 65536 multipath 0 1 alua 1 1 service-time 0 1 2 8:16 1 1
>>>>>>>>
>>>>>>>> The same test passes against kernel v4.20-rc6.
>>>>>>>
>>>>>>> What device is this being run on?
>>>>>>
>>>>>> I saw this issue on usb storage too.
>>>>>>
>>>>>> Seems it is introduced by commit ea86ea2cdced ("sbitmap: ammortize cost of
>>>>>> clearing bits"). When the IO hang happens, .cleared is 2, and .busy is 0 on
>>>>>> the sched_tag's sbitmap queue.
>>>>>
>>>>> You saw this running the same tests as Bart?
>>>>
>>>> Not the srp test as done by Bart, I just run 'parted' test on usb storage disk,
>>>> see the attached test script.
>>>>
>>>> Mostly it can be triggered in one run, sometimes it needs more.
>>>
>>> I'll take a look. The ->cleared doesn't make sense for QD=1, or on
>>> one word in general. But I'd like to try and understand why it hangs.
>>>
>>> Are you using a scheduler?
>>
>> OK, I think I see what it is, the shallow is missing the deferred clear.
>> On top of this, probably worth to check at what depths deferred starts
>> to make sense. For QD == 1, definitely not. But that should be on top of
>> the fix.
>>
>> Can you try this one?
>>
>>
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 2261136ae067..d98ba7af6bce 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -20,6 +20,47 @@
>>  #include <linux/sbitmap.h>
>>  #include <linux/seq_file.h>
>>  
>> +/*
>> + * See if we have deferred clears that we can batch move
>> + */
>> +static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
>> +{
>> +	unsigned long mask, val;
>> +	unsigned long __maybe_unused flags;
>> +	bool ret = false;
>> +
>> +	/* Silence bogus lockdep warning */
>> +#if defined(CONFIG_LOCKDEP)
>> +	local_irq_save(flags);
>> +#endif
>> +	spin_lock(&sb->map[index].swap_lock);
>> +
>> +	if (!sb->map[index].cleared)
>> +		goto out_unlock;
>> +
>> +	/*
>> +	 * First get a stable cleared mask, setting the old mask to 0.
>> +	 */
>> +	do {
>> +		mask = sb->map[index].cleared;
>> +	} while (cmpxchg(&sb->map[index].cleared, mask, 0) != mask);
>> +
>> +	/*
>> +	 * Now clear the masked bits in our free word
>> +	 */
>> +	do {
>> +		val = sb->map[index].word;
>> +	} while (cmpxchg(&sb->map[index].word, val, val & ~mask) != val);
>> +
>> +	ret = true;
>> +out_unlock:
>> +	spin_unlock(&sb->map[index].swap_lock);
>> +#if defined(CONFIG_LOCKDEP)
>> +	local_irq_restore(flags);
>> +#endif
>> +	return ret;
>> +}
>> +
>>  int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift,
>>  		      gfp_t flags, int node)
>>  {
>> @@ -70,6 +111,9 @@ void sbitmap_resize(struct sbitmap *sb, unsigned int depth)
>>  	unsigned int bits_per_word = 1U << sb->shift;
>>  	unsigned int i;
>>  
>> +	for (i = 0; i < sb->map_nr; i++)
>> +		sbitmap_deferred_clear(sb, i);
>> +
>>  	sb->depth = depth;
>>  	sb->map_nr = DIV_ROUND_UP(sb->depth, bits_per_word);
>>  
>> @@ -112,47 +156,6 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>>  	return nr;
>>  }
>>  
>> -/*
>> - * See if we have deferred clears that we can batch move
>> - */
>> -static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
>> -{
>> -	unsigned long mask, val;
>> -	unsigned long __maybe_unused flags;
>> -	bool ret = false;
>> -
>> -	/* Silence bogus lockdep warning */
>> -#if defined(CONFIG_LOCKDEP)
>> -	local_irq_save(flags);
>> -#endif
>> -	spin_lock(&sb->map[index].swap_lock);
>> -
>> -	if (!sb->map[index].cleared)
>> -		goto out_unlock;
>> -
>> -	/*
>> -	 * First get a stable cleared mask, setting the old mask to 0.
>> -	 */
>> -	do {
>> -		mask = sb->map[index].cleared;
>> -	} while (cmpxchg(&sb->map[index].cleared, mask, 0) != mask);
>> -
>> -	/*
>> -	 * Now clear the masked bits in our free word
>> -	 */
>> -	do {
>> -		val = sb->map[index].word;
>> -	} while (cmpxchg(&sb->map[index].word, val, val & ~mask) != val);
>> -
>> -	ret = true;
>> -out_unlock:
>> -	spin_unlock(&sb->map[index].swap_lock);
>> -#if defined(CONFIG_LOCKDEP)
>> -	local_irq_restore(flags);
>> -#endif
>> -	return ret;
>> -}
>> -
>>  static int sbitmap_find_bit_in_index(struct sbitmap *sb, int index,
>>  				     unsigned int alloc_hint, bool round_robin)
>>  {
>> @@ -215,6 +218,7 @@ int sbitmap_get_shallow(struct sbitmap *sb, unsigned int alloc_hint,
>>  	index = SB_NR_TO_INDEX(sb, alloc_hint);
>>  
>>  	for (i = 0; i < sb->map_nr; i++) {
>> +again:
>>  		nr = __sbitmap_get_word(&sb->map[index].word,
>>  					min(sb->map[index].depth, shallow_depth),
>>  					SB_NR_TO_BIT(sb, alloc_hint), true);
>> @@ -223,6 +227,9 @@ int sbitmap_get_shallow(struct sbitmap *sb, unsigned int alloc_hint,
>>  			break;
>>  		}
>>  
>> +		if (sbitmap_deferred_clear(sb, index))
>> +			goto again;
>> +
>>  		/* Jump to next index. */
>>  		index++;
>>  		alloc_hint = index << sb->shift;
> 
> Yeah, this one does work.

Good, thanks for testing. Can you try the next one too, just for good
measure? Then I can add your Tested-by.

-- 
Jens Axboe