On 1/15/21 05:24, Christoph Lameter wrote:
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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.
Some of those syscalls go through vdso, and don't enter the kernel --
nothing specific is necessary to allow them, and it would be pointless
and difficult to prevent them.
For syscalls that enter the kernel, it's often difficult to predict, if
they will or won't cause deferred processing, so I am afraid, it won't
be possible to provide a "safe" class of syscalls for this purpose and
not end up with something minimal like reading /sys and /proc. Right now
isolation only "allows" syscalls that exit isolation.
It may be possible to set up a filter by the system (allowing few safe
things like reading /proc) and let the user expand it by adding
combinations of syscall / file descriptor. If some device is known to
process operations safely, user can open it and mark file descriptor as
allowed, say, for reading.
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.
I think, it should be possible to process a syscall, and if any deferred
action occurred, exit isolation on return to userspace. Then there is a
question, how userspace should be notified about isolation being lost.
Normally this happens with a signal, but that is useful if we want
syscall to fail with EINTR, not to succeed. Make sure that signal
arrives after successful syscall return but before deferred action to
happen? Sounds convoluted. Maybe reflecting isolation status in vdso and
having the user check it there will be a good solution.
When I worked on my implementation I have encountered both a problem of
interaction with the rest of system from isolated task (at least simple
things as logging) and a problem of handling enter/exit from isolation
on a system when it's possible for a task to be interrupted early after
entering isolation due to various events that were still in progress on
other CPUs.
I ended up implementing a manager/helper task that talks to tasks over a
socket (when they are not isolated) and over ring buffers in shared
memory (when they are isolated). While the current implementation is
rather limited, the intention is to delegate to it everything that
isolated task either can't do at all (like, writing logs) or that it
would be cumbersome to implement (like monitoring the state of task,
determining presence of deferred work after the task returned to
userspace), etc.
It would be great if the complexity and amount of functionality of that
manager/helper task can be reduced, however I believe that having such a
task is a legitimate way of implementing things that otherwise would
require additional functionality in kernel.
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.
For my purposes breaking isolation on syscalls and notifications about
isolation breaking is a very useful approach -- this is why I kept it
exactly as it was in the original implementation by Chris Metcalf.
In applications that I intend to use isolation for, the primary concern
is consistent time for running code in userspace, so syscalls should be
only issued when the task is specifically not in isolated mode. If the
program issues a syscall by mistake (and that may happen when some
libraries are used, or thread synchronization primitives are kept from
non-isolated version of the program, even though isolated tasks are not
supposed to use those), it means not only that deferred work may cause
delay in the future, but also that there is an additional time to be
spent in kernel. This should be immediately visible to the developer,
and the best way to do it is by breaking isolation on syscall immediately.
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
There are many supposedly innocent things, nowhere at the scale of CPU
hotplug, that happen in a system and result in synchronization
implemented as an IPI to every online CPU. We should consider them to be
an ordinary occurrence, so there is a choice:
1. Ignore them completely and allow them in isolated mode. This will
delay userspace with no indication and no isolation breaking.
2. Allow them, and notify userspace afterwards (through vdso or through
userspace helper/manager over shared memory). This may be useful in
those rare situations when the consequences of delay can be mitigated
afterwards.
3. Make them break isolation, with userspace being notified normally
(ex: with a signal in the current implementation). I guess, can be used
if somehow most of the causes will be eliminated.
4. Prevent them from reaching the target CPU and make sure that whatever
synchronization they are intended to cause, will happen when intended
target CPU will enter to kernel later. Since we may have to synchronize
things like code modification, some of this synchronization has to
happen very early on kernel entry.
I am most interested in (4), so this is what was implemented in my
version of the patch (and currently I am trying to achieve completeness
and, if possible, elegance of the implementation).
I guess, if we want to add more controls, we can allow the user to
choose either of those four options, or of a subset of them. In my
opinion, if (4) will be available, and the only additional cost will be
time for synchronization spent in breaking isolation procedure, there is
not much need in the other three. Without (4) I don't think, the goal of
providing consistent, interruption-free environment is achieved at all,
so not implementing it would be very bad.
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.
Access to those is hardware/architecture-specific, and in many cases,
indeed, there is no need to issue a syscall at all.
However for many applications the model with a helper task performing
interactions with OS on a different core and exchanging data over shared
memory may be sufficient, and it will also provide clear separation
between operations that do require consistent timing and those that don't.
"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.
To be honest, I have no idea, what can and can not be tolerated by
applications other than what I am familiar with. Applications that I
know, require no interruptions at all, so I want to implement that. I
assume, someone already uses existing CPU isolation for the purpose of
providing "nearly interrupt-less" environment.
I can imaging something like a task of controlling a large slow-updating
LED display by bit-banging a strictly timed long serial message
representing a frame or frame update. If interrupted, it may, depending
on the protocol, corrupt the state of a single LED or fail to update
until the end of the screen, but the next start of message will reset
the state, and everything will work until the next interrupt. Maybe
there are more realistic or useful examples.
--
Alex
