Re: Re: [RFC 0/3] Improve memory statistics for virtio balloon

[zhenwei pi]

On 4/15/24 23:01, David Hildenbrand wrote:
> On 15.04.24 10:41, zhenwei pi wrote:
> > Hi,
> >
> > When the guest runs under critial memory pressure, the guest becomss
> > too slow, even sshd turns D state(uninterruptible) on memory
> > allocation. We can't login this VM to do any work on trouble shooting.
> >
> > Guest kernel log via virtual TTY(on host side) only provides a few
> > necessary log after OOM. More detail memory statistics are required,
> > then we can know explicit memory events and estimate the pressure.
> >
> > I'm going to introduce several VM counters for virtio balloon:
> > - oom-kill
> > - alloc-stall
> > - scan-async
> > - scan-direct
> > - reclaim-async
> > - reclaim-direct
>
> IIUC, we're only exposing events that are already getting provided via 
> all_vm_events(), correct?

Yes, all of these counters come from all_vm_events(). The 'alloc-stall' 
is summary of several classes of alloc-stall. please see '[RFC 2/3] 
virtio_balloon: introduce memory allocation stall counter'.

> In that case, I don't really see a major issue. Some considerations:
>
> (1) These new events are fairly Linux specific.
>
> PSWPIN and friends are fairly generic, but HGTLB is also already fairly 
> Linux specific already. OOM-kills don't really exist on Windows, for 
> example. We'll have to be careful of properly describing what the 
> semantics are.

I also notice FreeBSD supports virtio balloon for a long time, 'OOM 
kill' is used on FreeBSD too.(LINK: 
https://klarasystems.com/articles/exploring-swap-on-freebsd/)

> (2) How should we handle if Linux ever stops supporting a certain event 
> (e.g., major reclaim rework). I assume, simply return nothing like we 
> currently would for VIRTIO_BALLOON_S_HTLB_PGALLOC without 
> CONFIG_HUGETLB_PAGE.

Luckily, virtio balloon stats schema is tag-value style. This way would 
be safe enough.


Suggestions in patch [1-3] are good, I'll fix them in the next version 
if this series is acceptable.

--
zhenwei pi