Re: Patch "mm, vmstat: make quiet_vmstat lighter" has been added to the 4.4-stable tree

[Daniel Wagner <wagi@monom.org>]

Hi,

On 2019-12-11 15:46, Greg KH wrote:
> On Wed, Dec 11, 2019 at 10:32:49PM +0800, zhangyi (F) wrote:
>> We find a performance degradation under lmbench af_unix[1] test case after
>> mergeing this patch on my x86 qemu 4.4 machine. The test result is basically
>> stable for each teses.
>>
>> Host machine: CPU: Intel(R) Xeon(R) CPU E5-2690 v3
>>                CPU(s): 48
>>                MEM: 193047 MB
>>
>> Guest machine:  CPU: QEMU Virtual CPU version 2.5+
>>                  CPU(s): 8
>>                  MEM: 26065 MB
>>
>>    Before this patch:
>>    [root@localhost ~]# lmbench-3.0-a9/bin/x86_64-linux-gnu/lat_unix -P 1
>>    AF_UNIX sock stream latency: 133.7073 microseconds
>>
>>    After this patch:
>>    [root@localhost ~]# lmbench-3.0-a9/bin/x86_64-linux-gnu/lat_unix -P 1
>>    AF_UNIX sock stream latency: 156.4722 microseconds
>>
>> If we set task to a constant cpu, the degradation does not appear.
>>
>>    Before this patch:
>>    [root@localhost ~]# lmbench-3.0-a9/bin/x86_64-linux-gnu/lat_unix -P 1
>>    AF_UNIX sock stream latency: 17.9296 microseconds
>>
>>    After this patch:
>>    [root@localhost ~]# lmbench-3.0-a9/bin/x86_64-linux-gnu/lat_unix -P 1
>>    AF_UNIX sock stream latency: 17.7500 microseconds
>>
>> We also test it on the aarch64 hi1215 machine with 8 cpu cores.
>>
>>    Before this patch:
>>    [root@localhost ~]# ./lat_unix -P 1
>>    AF_UNIX sock stream latency: 30.7 microseconds
>>
>>    After this patch:
>>    [root@localhost ~]# ./lat_unix -P 1
>>    AF_UNIX sock stream latency: 37.5 microseconds
>>
>> Accessories included my reproduce config for x86 qemu. Any thoughts?
> 
> This fixes a bug, as reported by Daniel Wagner.  So it's probably better
> to have a stable system instead of a broken one, right?  :)
> 
> Daniel can provide more information if needed.

IIRC, this patch got necessary because  bdf3c006b9a2 ("vmstat: make 
vmstat_updater deferrable again and shut down on idle") was added to 
stable. Without it v4.4-rt is not working at all.

 > What about when you run your tests on a 4.9 or newer kernel that
 > already has this integrated?

That said, I was going through all changes in vmstat.c upstream and we 
backported almost all changes to v4.4 at that point. So if this is a 
really giving you a performance hit, I suspect you would see it upstream 
as well.

Thanks,
Daniel