Re: [RFC] tentative prctl task isolation interface

[Christoph Lameter <cl@linux.com>]

On Thu, 14 Jan 2021, Marcelo Tosatti wrote:

> > How does one do a oneshot flush of OS activities?
>
>         ret = prctl(PR_TASK_ISOLATION_REQUEST, ISOL_F_QUIESCE, 0, 0, 0);
>         if (ret == -1) {
>                 perror("prctl PR_TASK_ISOLATION_REQUEST");
>                 exit(0);
>         }
>
> >
> > I.e. I have a polling loop over numerous shared and I/o devices in user
> > space and I want to make sure that the system is quite before I enter the
> > loop.
>
> You could configure things in two ways: with syscalls allowed or not.

Well syscalls that do not cause deferred processing like getting the time
or determining the current cpu should be ok to use.

And I already said that I want the system to quiet down and allow system
calls. Some indication that deferred actions have occurred may be useful
by f.e. resetting the flag.

> 1) Add a new isolation feature ISOL_F_BLOCK_SYSCALLS (to block certain
> syscalls) along with ISOL_F_SETUP_NOTIF (to notify upon isolation
> breaking):

Well come up with a use case for that .... I know mine. What you propose
could be  useful for debugging for me but I would prefer the quiet down
approach where I determine when I use some syscalls or not and will deal
with the consequences.

>
> > Features that I think may be needed:
> >
> > F_ISOL_QUIESCE		-> quiet down now but allow all OS activities. OS
> > 			activites reset flag
> >
> > F_ISOL_BAREMETAL_HARD	-> No OS interruptions. Fault on syscalls that
> > 			require such actions in the future.
>
> Question: why BAREMETAL ?

To disinguish it from "Realtime". We want the processor for ourselves
without anything else running on it.

> Two comments:
>
> 1) HARD mode could also block activities from different CPUs that can
> interrupt this isolated CPU (for example CPU hotplug, or increasing
> per-CPU trace buffer size).

Blocking? The app should fail if any deferred actions are triggered as a
result of syscalls. It would give a warning with _WARN

> 2) For a type of application it is the case that certain interruptions
> can be tolerated, as long as they do not cross certain thresholds.
> For example, one loses the flexibility to read/write MSRs
> on the isolated CPUs (including performance counters,
> RDT/MBM type MSRs, frequency/power statistics) by
> forcing a "no interruptions" mode.

Does reading these really cause deferred actions by the OS? AFAICT you
could map these into memory as well as read them without OS activities.

"Interruptions that can be tolerated".... Well that is the wild west of
"realtime" where you can define how much of a time slice is "real" and how
much can be use by other processes. I do not think that any of that should
come into this API.