Re: [PATCH v2] bcache: set max writeback rate when I/O request is idle

[Stefan Priebe - Profihost AG <s.priebe@profihost.ag>]

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@vger.kernel.org #4.16+
>>> Signed-off-by: Coly Li <colyli@suse.de>
>>> Tested-by: Kai Krakow <kai@kaishome.de>
>>> Cc: Michael Lyle <mlyle@lyle.org>
>>> Cc: Stefan Priebe <s.priebe@profihost.ag>
>>
>> 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

> 
> Thanks.
> 
> Coly Li

Greets,
Stefan