Re: [PATCH v1] proc: Implement /proc/self/meminfo

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Jun 03, 2021 at 12:43:07PM +0200, legion@kernel.org wrote:
> From: Alexey Gladkov <legion@kernel.org>
> 
> The /proc/meminfo contains information regardless of the cgroups
> restrictions. This file is still widely used [1]. This means that all
> these programs will not work correctly inside container [2][3][4]. Some
> programs try to respect the cgroups limits, but not all of them
> implement support for all cgroup versions [5].
> 
> Correct information can be obtained from cgroups, but this requires the
> cgroups to be available inside container and the correct version of
> cgroups to be supported.
> 
> There is lxcfs [6] that emulates /proc/meminfo using fuse to provide
> information regarding cgroups. This patch can help them.
> 
> This patch adds /proc/self/meminfo that contains a subset of
> /proc/meminfo respecting cgroup restrictions.
> 
> We cannot just create /proc/self/meminfo and make a symlink at the old
> location because this will break the existing apparmor rules [7].
> Therefore, the patch adds a separate file with the same format.

Interesting work. Thanks. This is basically a variant of what I
suggested at Plumbers and in [1].

Judging from the patches sent by Waiman Long in [2] to also virtualize
/proc/cpuinfo and /sys/devices/system/cpu this is a larger push to
provide virtualized system information to containers.

Although somewhere in the thread here this veered off into apparently
just being a way for a process to gather information about it's own
resources. At which point I'm confused why looking at its cgroups
isn't enough.

So /proc/self/meminfo seems to just be the start. And note the two
approaches seem to diverge too. This provides a new file while the other
patchset virtualizes existing proc files/folders.

In any case it seems you might want to talk since afaict you're all at
the same company but don't seem to be aware of each others work (Which
happens of course.).

For the sake of history such patchsets have been pushed for before by
the Siteground people.

Chris and Johannes made a good point that the information provided in
this file can be gathered from cgroups already. So applications should
probably switch to reading those out of their cgroup and most are doing
that already.

And reading values out of cgroups is pretty straightforward even with
the differences between cgroup v1 and v2. Userspace is doing it all over
the place all of the time and the code has now existed for years so the
cgroup interface is a problem. And with cgroup v2 it keeps growing so
much more useful metrics that looking at meminfo isn't really cutting it
anyway.

So I think the argument that applications should start looking at their
cgroup info if they want to find out detailed info is a solid argument
that shouldn't be easily brushed aside.

What might be worth is knowing exactly what applications are looking at
/proc/meminfo and /proc/cpuinfo and make decision based on that info.
None of that is clearly outlined in the thread unfortunately.

So I immediately see two types of applications that could benefit from
this patchset. The first ones are legacy applications that aren't aware
of cgroups and aren't actively maintained. Introducing such
functionality for these applications seems a weak argument.

The second type is new and maintained applications that look at global
info such as /proc/meminfo and /proc/cpuinfo. So such applications have
ignored cgroups for a decade now. This makes it very unconvincing that
they will suddenly switch to a newly introduced file. Especially if the
entries in a new file aren't a 1:1 mapping of the old file.

Johannes made another good point about it not being clear what
applications actually want. And he's very right in that. It seems
straightforward to virtualize things like meminfo but it actually isn't.
And it's something you quite often discover after the fact. We have
extensive experience implementing it in LXCFS in userspace. People kept
and keep arguing what information exactly is supposed to go into
calculating those values based on what best helps their use-case.

Swap was an especially contentious point. In fact, sometimes users want
to turn of swap even though it exists on the host and there's a command
line switch in LXCFS to control that behavior.

Another example supporting Johannes worry is virtualizing /proc/cpuinfo
where some people wanted to virtualize cpu counts based on cpu shares.
So we have two modes to virtualize cpus: based on cpuset alone or based
on cpuset and cpu shares. And both modes are actively used. And that all
really depends on application and workload.

Finally, although LXCFS is briefly referenced in the commit message but
it isn't explained very well and what it does.

And we should consider it since this is a full existing userspace
solution to the problem solved in this patchset including Dan's JRE
use-case.

This is a project started in 2014 and it is in production use since 2014
and it delivers the features of this patchset here and more.

For example, it's used in the Linux susbystem of Chromebooks, it's used
by Alibaba (see [3]) and it is used for the JRE use-case by Google's
Anthos when migrating such legacy applications (see [4]).

At first, I was convinced we could make use of /proc/self/meminfo in
LXCFS which is why I held back but we can't. We can't simply bind-mount
it over /proc/meminfo because it's not a 1:1 correspondence between all
fields. We could potentially read some values we now calculate and
display it in /proc/meminfo but we can't stop virtualizing /proc/meminfo
itself. So we don't gain anything from this. When Alex asked me about it
I tried to come up with good ways to integrate this but the gain is just
too little for us.

Because our experience tells us that applications that want this type of
virtualization don't really care about heir own resources. They care
about a virtualized view of the system's resources. And the system in
question is often a container. But it get's very tricky since we don't
really define what a container is. So what data the user wants to see
depends on the used container runtime, type of container, and workload.
An application container has very different needs than a system
container that boots systemd. LXCFS can be very flexible here and
virtualize according to the users preferences (see the split between
cpuset and cpuset + cpu shares virtualization for cpu counts).

In any case, LXCFS is a tiny FUSE filesystem which virtualizes various
procfs and sysfs files for a container:

/proc/cpuinfo
/proc/diskstats
/proc/meminfo
/proc/stat
/proc/swaps
/proc/uptime
/proc/slabinfo
/sys/devices/system/cpu/*
/sys/devices/system/cpu/online

If you call top in a container that makes use of this it will display
everything virtualized to the container (See [5] for an example of
/proc/cpuinfo and /sys/devices/system/cpu/*.). And JRE will not
overallocate resources. It's actively used for all of that.

Below at [5] you can find an example where 2 cpus out of 8 have been
assigned to the container's cpuset. The container values are virtualized
as you can see.

[1]: https://lkml.org/lkml/2020/6/4/951
[2]: https://lore.kernel.org/lkml/YMe/cGV4JPbzFRk0@slm.duckdns.org
[3]: https://www.alibabacloud.com/blog/kubernetes-demystified-using-lxcfs-to-improve-container-resource-visibility_594109
[4]: https://cloud.google.com/blog/products/containers-kubernetes/migrate-for-anthos-streamlines-legacy-java-app-modernization
[5]: ## /proc/cpuinfo
     #### Host
     brauner@wittgenstein|~
     > ls -al /sys/devices/system/cpu/ | grep cpu[[:digit:]]
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu0
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu1
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu2
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu3
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu4
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu5
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu6
     drwxr-xr-x 10 root root    0 Jun 14 21:22 cpu7
     
     #### Container
     brauner@wittgenstein|~
     > lxc exec f1 -- ls -al /sys/devices/system/cpu/ | grep cpu[[:digit:]]
     drwxr-xr-x  2 nobody nogroup   0 Jun 15 10:22 cpu3
     drwxr-xr-x  2 nobody nogroup   0 Jun 15 10:22 cpu4
     
     ## /sys/devices/system/cpu/*
     #### Host
     brauner@wittgenstein|~
     > grep ^processor /proc/cpuinfo
     processor       : 0
     processor       : 1
     processor       : 2
     processor       : 3
     processor       : 4
     processor       : 5
     processor       : 6
     processor       : 7
     
     #### Container
     brauner@wittgenstein|~
     > lxc exec f1 -- grep ^processor /proc/cpuinfo
     processor       : 0
     processor       : 1

     ## top
     #### Host
     top - 13:16:47 up 15:54, 39 users,  load average: 0,76, 0,47, 0,40
     Tasks: 434 total,   1 running, 433 sleeping,   0 stopped,   0 zombie
     %Cpu0  :  2,7 us,  2,4 sy,  0,0 ni, 94,5 id,  0,0 wa,  0,0 hi,  0,3 si,  0,0 st
     %Cpu1  :  3,3 us,  1,3 sy,  0,0 ni, 95,3 id,  0,0 wa,  0,0 hi,  0,0 si,  0,0 st
     %Cpu2  :  1,6 us,  9,1 sy,  0,0 ni, 89,3 id,  0,0 wa,  0,0 hi,  0,0 si,  0,0 st
     %Cpu3  :  2,3 us,  1,3 sy,  0,0 ni, 96,4 id,  0,0 wa,  0,0 hi,  0,0 si,  0,0 st
     %Cpu4  :  2,7 us,  1,7 sy,  0,0 ni, 95,7 id,  0,0 wa,  0,0 hi,  0,0 si,  0,0 st
     %Cpu5  :  2,9 us,  2,9 sy,  0,0 ni, 94,1 id,  0,0 wa,  0,0 hi,  0,0 si,  0,0 st
     %Cpu6  :  2,3 us,  1,0 sy,  0,0 ni, 96,3 id,  0,0 wa,  0,0 hi,  0,3 si,  0,0 st
     %Cpu7  :  3,3 us,  1,3 sy,  0,0 ni, 95,4 id,  0,0 wa,  0,0 hi,  0,0 si,  0,0 st

     #### Container
     top - 11:16:13 up  2:08,  0 users,  load average: 0.27, 0.36, 0.36
     Tasks:  24 total,   1 running,  23 sleeping,   0 stopped,   0 zombie
     %Cpu0  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
     %Cpu1  :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st