Re: [PATCH V4 0/7] x86/intel_rdt: Intel Cache Allocation Technology

[Vikas Shivappa]

Hello Ingo/Peter,

On Thu, 26 Feb 2015, Ingo Molnar wrote:

> * Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>
> > > The CAT thing was annoying already, but at least one
> > > can find that in the SDM, this RDT thing, not a single
> > > mention.
> >
> > The problems of development at the bleeding edge. Would
> > you rather Linux sat on the sidelines until there are
> > enough Google hits from other users of new features?
>
> Well, we'd prefer there to be A) published documentation,
> or, lacking published documentation, there be B) a coherent
> technical description within the code itself what the
> purpose is and how it all works conceptually (minus the
> buzzwords), so that we have a common starting point when
> reviewing it.

Below is a short summary of the feature ( I have added a detailed 
documentation in 7/7 patch which fully explains the feature and usage ).

RDT (resource director technology) is the umbrella term which includes 
monitoring and enforcement of Processor shared resources. 
Matt's CMT patch takes care of cache monitoring :
https://lkml.kernel.org/r/1422038748-21397-1-git-send-email-matt@xxxxxxxxxxxxxxxxxxxx
The CAT patches support the cache enforcement part.

The cache allocation or enforcement takes place when applications fill L3 cache 
.Enforcement does not come into action if the cache is already filled and app 
just reads it.
The enforcement is done via MSR interface. One contains the ID(IA32_PQR_MSR) and 
other a cache bitmask. The bitmask represents one or more L3 cache ways.
The ID and the bitmask have a 1:1 mapping. When context switch takes place the 
scheduler just does an MSR write with the ID.

The cgroup would just provide an interface for the user to do the 
cache enforcement. Hence the cgroup is just used to partition the L3 cache 
resource. However this partition 
could be overlapping as the cache bitmasks can overlap. Eventually this cgroup 
can also be extended to support enforcement of more resources.
The cgroup interface has a 'cbm' file which represents a cgroup's 
bitmask.
The tasks belonging to a cgroup get to fill in the cache lines represented
by the cgroup's bitmask. Since there are a limited number of IDs available in 
the hardware, an error would be thrown when user ends up using all of them.


Thanks,
Vikas

> >    Technology: Intel Resource Director Technology
> >
> >    Description: Allows the hypervisor to monitor Last Level Cache usage at the application
> >    and VM levels.
> >
> >    Benefit: Helps to improve performance and efficiency by providing better
> >    information for scheduling, load balancing, and workload migration
> >
> > Which isn't any help in evaluating this patch series :-(
>
> No, but it already tells us more than the 0/7 description
> of the patch series did! It should be possible to improve
> on that.
>
> Maintainers reverse engineering the implementation is an
> inefficient approach.
>
> Thanks,
>
> 	Ingo
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