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Subject: Re: [PATCH v2 0/6] Improve visibility of writeback
To: Kent Overstreet <kent.overstreet@linux.dev>
Cc: akpm@linux-foundation.org, willy@infradead.org, jack@suse.cz,
 bfoster@redhat.com, tj@kernel.org, dsterba@suse.com, mjguzik@gmail.com,
 dhowells@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
 linux-fsdevel@vger.kernel.org
References: <20240327155751.3536-1-shikemeng@huaweicloud.com>
 <qyzaompqkxwdquqtofmqghvpi4m3twkrawn26rxs56aw4n2j3o@kt32f47dkjtu>
From: Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <6d83cf77-a0eb-3ee7-78f2-dae45562a6aa@huaweicloud.com>
Date: Wed, 3 Apr 2024 14:56:18 +0800
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on 3/29/2024 3:24 AM, Kent Overstreet wrote:
> On Wed, Mar 27, 2024 at 11:57:45PM +0800, Kemeng Shi wrote:
>> v1->v2:
>> -Send cleanup to wq_monitor.py separately.
>> -Add patch to avoid use after free of bdi.
>> -Rename wb_calc_cg_thresh to cgwb_calc_thresh as Tejun suggested.
>> -Use rcu walk to avoid use after free.
>> -Add debug output to each related patches.
>>
>> This series tries to improve visilibity of writeback. Patch 1 make
>> /sys/kernel/debug/bdi/xxx/stats show writeback info of whole bdi
>> instead of only writeback info in root cgroup. Patch 2 add a new
>> debug file /sys/kernel/debug/bdi/xxx/wb_stats to show per wb writeback
>> info. Patch 4 add wb_monitor.py to monitor basic writeback info
>> of running system, more info could be added on demand. Rest patches
>> are some random cleanups. More details can be found in respective
>> patches. Thanks!
>> This series is on top of patchset [1].
>>
>> [1] https://lore.kernel.org/lkml/20240123183332.876854-1-shikemeng@huaweicloud.com/T/#mc6455784a63d0f8aa1a2f5aff325abcdf9336b76
> 
> Not bad
> 
Hi Kent,
> I've been trying to improve our ability to debug latency issues - stalls
> of all sorts. While you're looking at all this code, do you think you
> could find some places to collect useful latency numbers?
I would like to do it to collect more useful info for writeback.
> 
> fs/bcachefs/time_stats.c has some code that's going to be moving out to
> lib/ at some point, after I switch it to MAD; if you could hook that up
> as well to a few points we could see at a glance if there are stalls
> happening in the writeback path.
I see that Tejun recommend to use bpf. I don't know much about bpf and
new approach in time_stats.c. For me personly, I think that it's better
to use the new approach after the work of moving code to lib/ is merged.
Then I would like to submit a patchset to discuss of using it in writeback.
Would this make sense to you. Look forward to your reply!

Thanks,
Kemeng
> 
>>
>> Following domain hierarchy is tested:
>>                 global domain (320G)
>>                 /                 \
>>         cgroup domain1(10G)     cgroup domain2(10G)
>>                 |                 |
>> bdi            wb1               wb2
>>
>> /* all writeback info of bdi is successfully collected */
>> # cat /sys/kernel/debug/bdi/252:16/stats:
>> BdiWriteback:              448 kB
>> BdiReclaimable:        1303904 kB
>> BdiDirtyThresh:      189914124 kB
>> DirtyThresh:         195337564 kB
>> BackgroundThresh:     32516508 kB
>> BdiDirtied:            3591392 kB
>> BdiWritten:            2287488 kB
>> BdiWriteBandwidth:      322248 kBps
>> b_dirty:                     0
>> b_io:                        0
>> b_more_io:                   2
>> b_dirty_time:                0
>> bdi_list:                    1
>> state:                       1
>>
>> /* per wb writeback info is collected */
>> # cat /sys/kernel/debug/bdi/252:16/wb_stats:
>> cat wb_stats
>> WbCgIno:                    1
>> WbWriteback:                0 kB
>> WbReclaimable:              0 kB
>> WbDirtyThresh:              0 kB
>> WbDirtied:                  0 kB
>> WbWritten:                  0 kB
>> WbWriteBandwidth:      102400 kBps
>> b_dirty:                    0
>> b_io:                       0
>> b_more_io:                  0
>> b_dirty_time:               0
>> state:                      1
>> WbCgIno:                 4284
>> WbWriteback:              448 kB
>> WbReclaimable:         818944 kB
>> WbDirtyThresh:        3096524 kB
>> WbDirtied:            2266880 kB
>> WbWritten:            1447936 kB
>> WbWriteBandwidth:      214036 kBps
>> b_dirty:                    0
>> b_io:                       0
>> b_more_io:                  1
>> b_dirty_time:               0
>> state:                      5
>> WbCgIno:                 4325
>> WbWriteback:              224 kB
>> WbReclaimable:         819392 kB
>> WbDirtyThresh:        2920088 kB
>> WbDirtied:            2551808 kB
>> WbWritten:            1732416 kB
>> WbWriteBandwidth:      201832 kBps
>> b_dirty:                    0
>> b_io:                       0
>> b_more_io:                  1
>> b_dirty_time:               0
>> state:                      5
>>
>> /* monitor writeback info */
>> # ./wb_monitor.py 252:16 -c
>>                   writeback  reclaimable   dirtied   written    avg_bw
>> 252:16_1                  0            0         0         0    102400
>> 252:16_4284             672       820064   9230368   8410304    685612
>> 252:16_4325             896       819840  10491264   9671648    652348
>> 252:16                 1568      1639904  19721632  18081952   1440360
>>
>>
>>                   writeback  reclaimable   dirtied   written    avg_bw
>> 252:16_1                  0            0         0         0    102400
>> 252:16_4284             672       820064   9230368   8410304    685612
>> 252:16_4325             896       819840  10491264   9671648    652348
>> 252:16                 1568      1639904  19721632  18081952   1440360
>> ...
>>
>> Kemeng Shi (6):
>>   writeback: protect race between bdi release and bdi_debug_stats_show
>>   writeback: collect stats of all wb of bdi in bdi_debug_stats_show
>>   writeback: support retrieving per group debug writeback stats of bdi
>>   writeback: add wb_monitor.py script to monitor writeback info on bdi
>>   writeback: rename nr_reclaimable to nr_dirty in balance_dirty_pages
>>   writeback: define GDTC_INIT_NO_WB to null
>>
>>  include/linux/writeback.h     |   1 +
>>  mm/backing-dev.c              | 203 ++++++++++++++++++++++++++++++----
>>  mm/page-writeback.c           |  31 ++++--
>>  tools/writeback/wb_monitor.py | 172 ++++++++++++++++++++++++++++
>>  4 files changed, 378 insertions(+), 29 deletions(-)
>>  create mode 100644 tools/writeback/wb_monitor.py
>>
>> -- 
>> 2.30.0
>>
>