Re: [PATCH v5 0/9] "Task_isolation" mode

Thomas Gleixner
2020-12-05T23:26:47Z

Pavel,

On Sat, Dec 05 2020 at 21:40, Pavel Machek wrote:
> So... what kind of guarantees does this aim to provide / what tasks it
> is useful for?
>
> For real time response, we have other approaches.

Depends on your requirements. Some problems are actually better solved
with busy polling. See below.

> If you want to guarantee performnace of the "isolated" task... I don't
> see how that works. Other tasks on the system still compete for DRAM
> bandwidth, caches, etc...

Applications which want to run as undisturbed as possible. There is
quite a range of those:

- Hardware in the loop simulation is today often done with that crude
approach of "offlining" a CPU and then instead of playing dead
jumping to a preloaded bare metal executable. That's a horrible hack
and impossible to debug, but gives them the results they need to
achieve. These applications are well optimized vs. cache and memory
foot print, so they don't worry about these things too much and they
surely don't run on SMI and BIOS value add inflicted machines.

Don't even think about waiting for an interrupt to achieve what
these folks are doing. So no, there are problems which a general
purpose realtime OS cannot solve ever.

- HPC computations on large data sets. While the memory foot print is
large the access patterns are cache optimized.

The problem there is that any unnecessary IPI, tick interrupt or
whatever nuisance is disturbing the carefully optimized cache usage
and alone getting rid of the timer interrupt gained them measurable
performance. Even very low single digit percentage of runtime saving
is valuable for these folks because the compute time on such beasts
is expensive.

- Realtime guests in KVM. With posted interrupts and a fully populated
host side page table there is no point in running host side
interrupts or IPIs for random accounting or whatever purposes as
they affect the latency in the guest. With all the side effects
mitigated and a properly set up guest and host it is possible to get
to a zero exit situation after the bootup phase which means pretty
much matching bare metal behaviour.

Yes, you can do that with e.g. Jailhouse as well, but you lose lots
of the fancy things KVM provides. And people care about these not
just because they are fancy. They care because their application
scenario needs them.

There are more reasons why people want to be able to get as much
isolation from the OS as possible but at the same time have a sane
execution environment, debugging, performance monitoring and the OS
provided protection mechanisms instead of horrible hacks.

Isolation makes sense for a range of applications and there is no reason
why Linux should not support them.

Thanks,

tglx