# Re: [PATCH 2/5] mm: correct calculation of cgroup wb's bg_thresh in wb_over_bg_thresh

From: Kemeng Shi
Date: Thu Feb 08 2024 - 04:29:25 EST

on 1/30/2024 5:00 AM, Tejun Heo wrote:
> Hello,
>
> On Wed, Jan 24, 2024 at 10:01:47AM +0800, Kemeng Shi wrote:
>> Hi Tejun, thanks for reply. For cgroup wb, it will belongs to a global wb
>> domain and a cgroup domain. I agree the way how we calculate wb's threshold
>> in global domain as you described above. This patch tries to fix calculation
>> of wb's threshold in cgroup domain which now is wb_calc_thresh(mdtc->wb,
>> mdtc->bg_thresh)), means:
>> (wb bandwidth) / (system bandwidth) * (*cgroup domain threshold*)
>> The cgroup domain threshold is
>> (memory of cgroup domain) / (memory of system) * (system threshold).
>> Then the wb's threshold in cgroup will be smaller than expected.
>>
>> Consider following domain hierarchy:
>> global domain (100G)
>> / \
>> cgroup domain1(50G) cgroup domain2(50G)
>> | |
>> bdi wb1 wb2
>> Assume wb1 and wb2 has the same bandwidth.
>> We have global domain bg_thresh 10G, cgroup domain bg_thresh 5G.
>> Then we have:
>> wb's thresh in global domain = 10G * (wb bandwidth) / (system bandwidth)
>> = 10G * 1/2 = 5G
>> wb's thresh in cgroup domain = 5G * (wb bandwidth) / (system bandwidth)
>> = 5G * 1/2 = 2.5G
>> At last, wb1 and wb2 will be limited at 2.5G, the system will be limited
>> at 5G which is less than global domain bg_thresh 10G.
>>
>> After the fix, threshold in cgroup domain will be:
>> (wb bandwidth) / (cgroup bandwidth) * (cgroup domain threshold)
>> The wb1 and wb2 will be limited at 5G, the system will be limited at
>> 10G which equals to global domain bg_thresh 10G.
>>
>> As I didn't take a deep look into memory cgroup, please correct me if
>> anything is wrong. Thanks!
>>> Also, how is this tested? Was there a case where the existing code
>>> misbehaved that's improved by this patch? Or is this just from reading code?
>>
>> This is jut from reading code. Would the case showed above convince you
>
> So, the explanation makes some sense to me but can you please construct a
> case to actually demonstrate the problem and fix? I don't think it'd be wise
> to apply the change without actually observing the code change does what it
> says it does.
Hi Tejun, sorry for the delay as I found there is a issue that keep triggering
writeback even the dirty page is under dirty background threshold. The issue
make it difficult to observe the expected improvment from this patch. I try to
fix it in [1] and test this patch based on the fix patches.
Run test as following:

/* make background writeback easier to observe */
echo 300000 > /proc/sys/vm/dirty_expire_centisecs
echo 100 > /proc/sys/vm/dirty_writeback_centisecs
/* enable memory and io cgroup */
echo "+memory +io" > /sys/fs/cgroup/cgroup.subtree_control

/* run fio in group1 with shell */
cd /sys/fs/cgroup
mkdir group1
cd group1
echo 10G > memory.high
echo 10G > memory.max
echo \$\$ > cgroup.procs
mkfs.ext4 -F /dev/vdb
mount /dev/vdb /bdi1/
fio -name test -filename=/bdi1/file -size=800M -ioengine=libaio -bs=4K -iodepth=1 -rw=write -direct=0 --time_based -runtime=60 -invalidate=0

/* run another fio in group2 with another shell */
cd /sys/fs/cgroup
mkdir group2
cd group2
echo 10G > memory.high
echo 10G > memory.max
echo \$\$ > cgroup.procs
mkfs.ext4 -F /dev/vdc
mount /dev/vdc /bdi2/
fio -name test -filename=/bdi2/file -size=800M -ioengine=libaio -bs=4K -iodepth=1 -rw=write -direct=0 --time_based -runtime=60 -invalidate=0

Before the fix we got (result of three tests):
fio1
WRITE: bw=1304MiB/s (1367MB/s), 1304MiB/s-1304MiB/s (1367MB/s-1367MB/s), io=76.4GiB (82.0GB), run=60001-60001msec
WRITE: bw=1351MiB/s (1417MB/s), 1351MiB/s-1351MiB/s (1417MB/s-1417MB/s), io=79.2GiB (85.0GB), run=60001-60001msec
WRITE: bw=1373MiB/s (1440MB/s), 1373MiB/s-1373MiB/s (1440MB/s-1440MB/s), io=80.5GiB (86.4GB), run=60001-60001msec
fio2
WRITE: bw=1134MiB/s (1190MB/s), 1134MiB/s-1134MiB/s (1190MB/s-1190MB/s), io=66.5GiB (71.4GB), run=60001-60001msec
WRITE: bw=1414MiB/s (1483MB/s), 1414MiB/s-1414MiB/s (1483MB/s-1483MB/s), io=82.8GiB (88.0GB), run=60001-60001msec
WRITE: bw=1469MiB/s (1540MB/s), 1469MiB/s-1469MiB/s (1540MB/s-1540MB/s), io=86.0GiB (92.4GB), run=60001-60001msec

After the fix we got (result of three tests):
fio1
WRITE: bw=1719MiB/s (1802MB/s), 1719MiB/s-1719MiB/s (1802MB/s-1802MB/s), io=101GiB (108GB), run=60001-60001msec
WRITE: bw=1723MiB/s (1806MB/s), 1723MiB/s-1723MiB/s (1806MB/s-1806MB/s), io=101GiB (108GB), run=60001-60001msec
WRITE: bw=1691MiB/s (1774MB/s), 1691MiB/s-1691MiB/s (1774MB/s-1774MB/s), io=99.2GiB (106GB), run=60036-60036msec
fio2
WRITE: bw=1692MiB/s (1774MB/s), 1692MiB/s-1692MiB/s (1774MB/s-1774MB/s), io=99.1GiB (106GB), run=60001-60001msec
WRITE: bw=1681MiB/s (1763MB/s), 1681MiB/s-1681MiB/s (1763MB/s-1763MB/s), io=98.5GiB (106GB), run=60001-60001msec
WRITE: bw=1671MiB/s (1752MB/s), 1671MiB/s-1671MiB/s (1752MB/s-1752MB/s), io=97.9GiB (105GB), run=60001-60001msec
I also add code to print the pages written in writeback and pages written in
writeback reduce a lot and are rare after this fix.

[1] https://lore.kernel.org/linux-fsdevel/20240208172024.23625-2-shikemeng@xxxxxxxxxxxxxxx/T/#u
>
> Thanks.
>