Re: [PATCH v2 0/6] Improve visibility of writeback

[Kent Overstreet]

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@xxxxxxxxxxxxxxx/T/#mc6455784a63d0f8aa1a2f5aff325abcdf9336b76

Not bad

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?

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.

> 
> 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
>