I don't remember how much I answered this email, but I need to finish that :-)
On Fri, Apr 08, 2016 at 12:34:48PM -0400, Chris Metcalf wrote:
On 4/8/2016 9:56 AM, Frederic Weisbecker wrote:Good, although that quiescing on kernel return must be an option.
On Wed, Mar 09, 2016 at 02:39:28PM -0500, Chris Metcalf wrote:So what happens if an interrupt does occur?
TL;DR: Let's make an explicit decision about whether task isolationBut then in this mode, what happens when an interrupt triggers.
should be "persistent" or "one-shot". Both have some advantages.
An important high-level issue is how "sticky" task isolation mode is.
We need to choose one of these two options:
"Persistent mode": A task switches state to "task isolation" mode
(kind of a level-triggered analogy) and stays there indefinitely. It
can make a syscall, take a page fault, etc., if it wants to, but the
kernel protects it from incurring any further asynchronous interrupts.
This is the model I've been advocating for.
In the "base" task isolation mode, you just take the interrupt, then
wait to quiesce any further kernel timer ticks, etc., and return to
the process. This at least limits the damage to being a single
interruption rather than potentially additional ones, if the interrupt
also caused timers to get queued, etc.
If you enable "strict" mode, we disable task isolation mode for thatGood.
core and deliver a signal to it. This lets the application know that
an interrupt occurred, and it can take whatever kind of logging or
debugging action it wants to, re-enable task isolation if it wants to
and continue, or just exit or abort, etc.
If you don't enable "strict" mode, but you do haveRight, I suggest we use trace events btw.
task_isolation_debug enabled as a boot flag, you will at least get a
console dump with a backtrace and whatever other data we have.
(Sometimes the debug info actually includes a backtrace of the
interrupting core, if it's an IPI or TLB flush from another core,
which can be pretty useful.)
Yes.Hmm, so I think I'm not really understanding what you are suggesting."One-shot mode": A task requests isolation via prctl(), the kernelNo I think we can issue syscalls for exemple. But asynchronous interruptions
ensures it is isolated on return from the prctl(), but then as soon as
it enters the kernel again, task isolation is switched off until
another prctl is issued. This is what you recommended in your last
such as exceptions (actually somewhat synchronous but can be unexpected) and
interrupts are what we want to avoid.
We're certainly in agreement that avoiding interrupts and exceptions
is important. I'm arguing that the way to deal with them is to
generate appropriate signals/printks, etc.
I'm not actually sure whatThey are not all deterministic. For example a breakpoint, a step, a trap
you're recommending we do to avoid exceptions. Since they're
synchronous and deterministic, we can't really avoid them if the
program wants to issue them. For example, mmap() some anonymous
memory and then start running, and you'll take exceptions each time
you touch a page in that mapped region. I'd argue it's an application
bug; one should enable "strict" mode to catch and deal with such bugs.
can be set up by another process. So this is not entirely under the control
of the user.
(Typically the recommendation is to do an mlockall() before startingNow how do you determine which exception is a bug and which is expected?
task isolation mode, to handle the case of page faults. But you can
do that and still be screwed by another thread in your process doing a
fork() and then your pages end up read-only for COW and you have to
fault them back in. But, that's an application bug for a
task-isolation thread, and should just be treated as such.)
Strict mode should refuse all of them.
Well, hard isolation is what I would call strict mode.No, it's still hard isolation. The distinction is that we offer a wayThere are a number of pros and cons to the two models. I think onSo this is not hard isolation anymore. This is rather soft isolation with
balance I still like the "persistent mode" approach, but here's all
the pros/cons I can think of:
PRO for persistent mode: A somewhat easier programming model. Users
can just imagine "task isolation" as a way for them to still be able
to use the kernel exactly as they always have; it's just slower to get
back out of the kernel so you use it judiciously. For example, a
process is free to call write() on a socket to perform a diagnostic,
but when returning from the write() syscall, the kernel will hold the
task in kernel mode until any timer ticks (perhaps from networking
stuff) are complete, and then let it return to userspace to continue
in task isolation mode.
best efforts to avoid disturbance.
to get in and out of the kernel "safely" if you want to run in that
mode. The syscalls can take a long time if the syscall ends up
requiring some additional timer ticks to finish sorting out whatever
it was you asked the kernel to do, but once you're back in userspace
you immediately regain "hard" isolation. It's under program control.
Or, you can enable "strict" mode, and then you get hard isolation
without the ability to get in and out of the kernel at all: the kernel
just kills you if you try to leave hard isolation other than by an
Right.Surely we can have different levels of isolation.Well, we have nohz_full now, and by adding task-isolation, we have
two. Or three if you count "base" and "strict" mode task isolation as
two separate levels.
Indeed :-) Well if it's a task property, we need to handle its affinity properly then.I'm still wondering what to do if the task migrates to another CPU. In fact,Well, we did go around on this issue once already (last August) and at
perhaps what you're trying to do is rather a CPU property than a
the time you were encouraging isolation to be a "task" property, not a
You convinced me at the time :-)
You're right that migration conflicts with task isolation. ButYes.
certainly, if a task has enabled "strict" semantics, it can't migrate;
it will lose task isolation entirely and get a signal instead,
regardless of whether it calls sched_setaffinity() on itself, or if
someone else changes its affinity and it gets a kick.
However, if a task doesn't have strict mode enabled, it can callThat doesn't look sane. The user asks the kernel to get away as much
sched_setaffinity() and force itself onto a non-task_isolation cpu and
it won't get any isolation until it schedules itself back onto a
task_isolation cpu, at which point it wakes up on the new cpu with
hard isolation still in effect. I can make up reasons why this sort
of thing might be useful, but it's probably a corner case.
as it can but if we are in a non-nohz-full CPU we know we can't provide that
service (or rather that non-service).
So we would refuse to enter in task isolation mode if it doesn't run in a
full dynticks CPUs whereas we accept that it migrates later to a periodic
CPU?. This isn't consistent.
However, this makes me wonder if "strict" mode should be the defaultWell we can't really implement that strict mode until we fix the 1Hz issue, right?
for task isolation?? That way task isolation really doesn't conflict
semantically with migration. And we could provide a "weak" mode, or a
"kernel-friendly" mode, or some such nomenclature, and define the
migration semantics just for that case, where it makes it clear it's a
Besides, is this something that anyone needs now?
So those workloads couldn't stand an interrupt? Like they would like a signalI think I heard about workloads that need such strict hard isolation.Certainly HFT is one case.
Workloads that really can not afford any disturbance. They even
use userspace network stack. Maybe HFT?
A lot of TILE-Gx customers using task isolation (which we call
"dataplane" or "Zero-Overhead Linux") are doing high-speed network
applications with user-space networking stacks. It can be DPDK, or it
can be another TCP/IP stack (we ship one called tStack) or it
could just be an application directly messing with the network
hardware from userspace. These are exactly the applications that led
me into this part of kernel development in the first place.
Googling "Zero-Overhead Linux" does take you to some discussions
of customers that have used this functionality.
and exit the strict mode if it happens?
I think that we need to wait for somebody who explicitly request that feature
before we work on it, so we get sure the semantics really agree with someone's
real load case.
It's not in the patch series, I'm talking about the strict mode :-)I'm not sure what part of the patch series you're saying you don'tI think we can actually make both modes available to users with justI think we shouldn't bother with strict hard isolation if we don't need
another flag bit, so maybe we can look at what that looks like in v11:
adding a PR_TASK_ISOLATION_ONESHOT flag would turn off task
isolation at the next syscall entry, page fault, etc. Then we can
think more specifically about whether we want to remove the flag or
not, and if we remove it, whether we want to make the code that was
controlled by it unconditionally true or unconditionally false
(i.e. remove it again).
it yet. The implementation may well be invasive. Lets wait for someone
who really needs it.
think we need yet. I'd argue the whole patch series is "hard
isolation", and that the "strict" mode introduced in patch 06/13 isn't
Ok, so thinking about that talk, I'm wondering if we need some flagsSo your requirements are actually hard isolation but in userspace?Yes, exactly. Were you thinking about a kernel-level hard isolation?
That would have some similarities, I guess, but in some ways might
actually be a harder problem.
And what happens if you get interrupted in userspace? What about pageSee above :-)
faults and other exceptions?
I hope we're converging here. If you want to talk live or chat online
to help finish converging, perhaps that would make sense? I'd be
happy to take notes and publish a summary of wherever we get to.
Thanks for taking the time to review this!
Strict mode would be the three above OR'ed. It's just some random thoughts
but that would help define which level of kernel intrusion the user is ready
I'm just not sure how granular we want that interface to be.