Re: [PATCH 0/6] support "dataplane" mode for nohz_full

Chris Metcalf
2015-05-11T19:54:51Z

(Oops, resending and forcing html off.)

On 05/09/2015 03:19 AM, Andy Lutomirski wrote:
> Naming aside, I don't think this should be a per-task flag at all. We
> already have way too much overhead per syscall in nohz mode, and it
> would be nice to get the per-syscall overhead as low as possible. We
> should strive, for all tasks, to keep syscall overhead down*and*
> avoid as many interrupts as possible.
>
> That being said, I do see a legitimate use for a way to tell the
> kernel "I'm going to run in userspace for a long time; stay away".
> But shouldn't that be a single operation, not an ongoing flag? IOW, I
> think that we should have a new syscall quiesce() or something rather
> than a prctl.

Yes, if all you are concerned about is quiescing the tick, we could
probably do it as a new syscall.

I do note that you'd want to try to actually do the quiesce as late as
possible - in particular, if you just did it in the usual syscall, you
might miss out on a timer that is set by softirq, or even something
that happened when you called schedule() on the syscall exit path.
Doing it as late as we are doing helps to ensure that that doesn't
happen. We could still arrange for this semantics by having a new
quiesce() syscall set a temporary task bit that was cleared on
return to userspace, but as you pointed out in a different email,
that gets tricky if you end up doing multiple user_exit() calls on
your way back to userspace.

More to the point, I think it's actually important to know when an
application believes it's in userspace-only mode as an actual state
bit, rather than just during its transitional moment. If an
application calls the kernel at an unexpected time (third-party code
is the usual culprit for our customers, whether it's syscalls, page
faults, or other things) we would prefer to have the "quiesce"
semantics stay in force and cause the third-party code to be
visibly very slow, rather than cause a totally unexpected and
hard-to-diagnose interrupt show up later as we are still going
around the loop that we thought was safely userspace-only.

And, for debugging the kernel, it's crazy helpful to have that state
bit in place: see patch 6/6 in the series for how we can diagnose
things like "a different core just queued an IPI that will hit a
dataplane core unexpectedly". Having that state bit makes this sort
of thing a trivial check in the kernel and relatively easy to debug.

Finally, I proposed a "strict" mode in patch 5/6 where we kill the
process if it voluntarily enters the kernel by mistake after saying it
wasn't going to any more. To do this requires a state bit, so
carrying another state bit for "quiesce on user entry" seems pretty
reasonable.

--
Chris Metcalf, EZChip Semiconductor