Re: [PATCH v2] mm: Add configuration to control whether vmpressure notifier is enabled

[yong w <yongw.pur@gmail.com>]

Michal Hocko <mhocko@suse.com> 于2021年8月20日周五 下午7:26写道：
>
> On Thu 19-08-21 16:53:39, yongw.pur@gmail.com wrote:
> > From: wangyong <wang.yong@zte.com.cn>
> >
> > Inspired by PSI features, vmpressure inotifier function should
> > also be configured to decide whether it is used, because it is an
> > independent feature which notifies the user of memory pressure.
>
> Yes, it is an independent feature indeed but what is the actual reason
> to put a more configuration space here. Config options are not free both
> from the user experience POV as well as the code maintenance. Why do we
> need to disable this feature. Who can benefit from such a setup?
>
> > So we add configuration to control whether vmpressure notifier is
> > enabled, and provide a boot parameter to use vmpressure notifier
> > flexibly.
>
> Flexibility is nice but not free as mentioned above.
>
> > Use Christoph Lamenter’s pagefault tool
> > (https://lkml.org/lkml/2006/8/29/294) for comparative testing.
> > Test with 5.14.0-rc5-next-20210813 on x86_64 4G Ram
> > To ensure that the vmpressure function is executed, we enable zram
> > and let the program occupy memory so that some memory is swapped out
> >
> > unpatched:
> > Gb    Rep     Thr     CLine   User(s) System(s) Wall(s) flt/cpu/s     fault/wsec
> > 2     1       1       1       0.1     0.97    1.13    485490.062      463533.34
> > 2     1       1       1       0.11    0.96    1.12    483086.072      465309.495
> > 2     1       1       1       0.1     0.95    1.11    496687.098      469887.643
> > 2     1       1       1       0.09    0.97    1.11    489711.434      468402.102
> > 2     1       1       1       0.13    0.94    1.12    484159.415      466080.941
> > average                               0.106   0.958   1.118   487826.8162     466642.7042
> >
> > patched and CONFIG_MEMCG_VMPRESSURE is not set:
> > Gb    Rep     Thr     CLine   User(s) System(s) Wall(s) flt/cpu/s     fault/wsec
> > 2     1       1       1       0.1     0.96    1.1     490942.682      473125.98
> > 2     1       1       1       0.08    0.99    1.13    484987.521      463161.975
> > 2     1       1       1       0.09    0.96    1.09    498824.98       476696.066
> > 2     1       1       1       0.1     0.97    1.12    484127.673      465951.238
> > 2     1       1       1       0.1     0.97    1.11    487032          468964.662
> > average                               0.094   0.97    1.11    489182.9712     469579.9842
> >
> > According to flt/cpu/s, performance improved by 0.2% which is not obvious.
>
> I haven't checked how are those numbers calculated but from a very brief
> look it seems like the variation between different runs is higher than
> 0.2%. Have you checked the average against standard deviation to get a
> better idea whether the difference is really outside of the noise?
> --
> Michal Hocko
> SUSE Labs

Thanks for your reply.
The reason for adding configuration is as follows：
1. Referring to [PATCH] psi: make disabling/enabling easier for vendor
kernels, the modification
is also applicable to vmpressure.

2. With the introduction of psi into the kernel, there are two memory
pressure monitoring methods，
it is not necessary to use both and it makes sense to make vmpressure
configurable.

3. In the case where the user does not need vmpressure,  vmpressure
calculation is additional overhead.
In some special scenes with tight memory, vmpressure will be executed
frequently.we use "likely" and "inline"
to improve the performance of the kernel, why not reduce some
unnecessary calculations?

4. This patch is forward compatible, because VMPRESSURE is set by
default and user does not need to
make any changes. For users who do not need to use vmpressure notifier
or users who use psi, they
can choose not to configure this function.

Thanks.