On 2018/7/24 4:33 PM, Stefan Priebe - Profihost AG wrote:
> Am 24.07.2018 um 10:28 schrieb Coly Li:
>> On 2018/7/24 3:16 PM, Stefan Priebe - Profihost AG wrote:
>>> Am 24.07.2018 um 06:03 schrieb Coly Li:
>>>> Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
>>>> allows the writeback rate to be faster if there is no I/O request on a
>>>> bcache device. It works well if there is only one bcache device attached
>>>> to the cache set. If there are many bcache devices attached to a cache
>>>> set, it may introduce performance regression because multiple faster
>>>> writeback threads of the idle bcache devices will compete the btree level
>>>> locks with the bcache device who have I/O requests coming.
>>>>
>>>> This patch fixes the above issue by only permitting fast writebac when
>>>> all bcache devices attached on the cache set are idle. And if one of the
>>>> bcache devices has new I/O request coming, minimized all writeback
>>>> throughput immediately and let PI controller __update_writeback_rate()
>>>> to decide the upcoming writeback rate for each bcache device.
>>>>
>>>> Also when all bcache devices are idle, limited wrieback rate to a small
>>>> number is wast of thoughput, especially when backing devices are slower
>>>> non-rotation devices (e.g. SATA SSD). This patch sets a max writeback
>>>> rate for each backing device if the whole cache set is idle. A faster
>>>> writeback rate in idle time means new I/Os may have more available space
>>>> for dirty data, and people may observe a better write performance then.
>>>>
>>>> Please note bcache may change its cache mode in run time, and this patch
>>>> still works if the cache mode is switched from writeback mode and there
>>>> is still dirty data on cache.
>>>>
>>>> Fixes: Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
>>>> Cc: stable@xxxxxxxxxxxxxxx #4.16+
>>>> Signed-off-by: Coly Li <colyli@xxxxxxx>
>>>> Tested-by: Kai Krakow <kai@xxxxxxxxxxx>
>>>> Cc: Michael Lyle <mlyle@xxxxxxxx>
>>>> Cc: Stefan Priebe <s.priebe@xxxxxxxxxxxx>
>>>
>>> Hi Coly,
>>>
>>> i'm still experiencing the same bug as yesterday while rebooting every
>>> two times i get a deadlock in cache_set_free.
>>>
>>> Sadly it's so late ion the shutdown process that netconsole is already
>>> unloaded or at least i get no messages from while it happens. The traces
>>> look the same like yesterday.
>>
>> Hi Stefan,
>>
>> Do you use the v2 patch on latest SLE15 kernel source?
>
> Yes - i use the kernel code from here:
> https://github.com/openSUSE/kernel-source/commits/SLE15
>
>> Let me try to
>> reproduce it on my machine. I assume when you reboot, the cache is still
>> dirty and not clean up, am I right ?
>
> Yes with around 15GB of dirty data - ceph is running on top of it and
> generated many random writes.
>
>> And which file system do you mount
>> on top of the bcache device ?
> XFS
Hi Stefan,
Thanks for the information. I managed to reproduce a similar deadlock on
my small box. Let me see how to fix it and post a new version ASAP :-)
Coly Li
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