Re: [PATCH 1/2] cgroup/dmem: add per-region event counters

[Natalie Vock]

Hi,

On 6/25/26 04:10, Hongfu Li wrote:
> Hi, Tejun
> Thanks for the review comments.
>
> > > Add dmem.events to report hierarchical low/max event counts per DMEM
> > > region.  Increment counters on dmem.max allocation failures and
> > > dmem.low protection events.  The file is available for non-root cgroups
> > > only.
> >
> > Please don't double space in descs or comments. Also, maybe it's obvious but
> > it'd help if you list why and how this is useful. Why do we want to add
> > this?
>
> I'll fix the double spacing in the commit message and comments.
>
> As for the motivation: dmem already exposes per-region limits and current
> usage, but not how often those limits actually matter at runtime. Without
> event counters, it's hard to tell whether allocation failures come from
> this cgroup, a parent limit, or pressure elsewhere in the hierarchy.
> dmem.events provides that visibility for tuning dmem.low/dmem.max and
> diagnosing recurring device memory pressure.

Shouldn't you be able to deduce this rather trivially from just looking 
at the current usage together with the low/max limits you already set? 
I'm not sure I really see anything this events file provides that 
analysis of current usage and set limits doesn't? If your usage is 
highly variable, the separately-developed dmem.peak file might also suit 
your needs, but still, not sure what you can do with dmem.events that 
you can't already do with these tools.

Best,
Natalie

> I'll expand the commit message to cover this.
>   
> > > +  dmem.events
> > > +	A read-only file that reports the number of times each cgroup
> > > +	has hit its configured memory limits.  The format lists each
> > > +	region on a single line, followed by the event counters::
> > > +
> > > +	  drm/0000:03:00.0/vram0 low 0 max 3
> > > +	  drm/0000:03:00.0/stolen low 0 max 0
> >
> > This isn't a supported file format. Please read the documentation on allowed
> > formats.
>
> Thanks for catching this. I'll switch dmem.events to nested-keyed format (region low=N max=M).
>
> Thanks again for the valuable feedback.
>
> Best regards,
> Hongfu