Re: [PATCH RFC] sched: deferred set priority (dprio)

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Sat, 2014-07-26 at 11:30 -0700, Sergey Oboguev wrote: 
> On Sat, Jul 26, 2014 at 1:58 AM, Mike Galbraith
> <umgwanakikbuti@gmail.com> wrote:
> > On Fri, 2014-07-25 at 12:45 -0700, Sergey Oboguev wrote:
> >> [This is a repost of the message from few day ago, with patch file
> >> inline instead of being pointed by the URL.]
> >>
> >> This patch is intended to improve the support for fine-grain parallel
> >> applications that may sometimes need to change the priority of their threads at
> >> a very high rate, hundreds or even thousands of times per scheduling timeslice.
> >>
> >> These are typically applications that have to execute short or very short
> >> lock-holding critical or otherwise time-urgent sections of code at a very high
> >> frequency and need to protect these sections with "set priority" system calls,
> >> one "set priority" call to elevate current thread priority before entering the
> >> critical or time-urgent section, followed by another call to downgrade thread
> >> priority at the completion of the section. Due to the high frequency of
> >> entering and leaving critical or time-urgent sections, the cost of these "set
> >> priority" system calls may raise to a noticeable part of an application's
> >> overall expended CPU time. Proposed "deferred set priority" facility allows to
> >> largely eliminate the cost of these system calls.
> >
> > So you essentially want to ship preempt_disable() off to userspace?
> >
> 
> Only to the extent preemption control is already exported to the userspace and
> a task is already authorized to control its preemption by its RLIMIT_RTPRIO,
> RLIMIT_NICE and capable(CAP_SYS_NICE).
> 
> DPRIO does not amplify a taks's capability to elevate its priority and block
> other tasks, it just reduces the computational cost of frequest
> sched_setattr(2) calls.

Exactly.  You are abusing realtime, and you are not the only guy out
there doing that.  What you want is control over a userspace critical
section, and you are willing to do whatever is necessary to get that.  I
think your code is a really good example of how far people are willing
to go, but I hope this goes nowhere beyond getting people to think about
what you and others want.

I would say cut to the chase, if what you want/need is a privileged
userspace lock, make one, and put _all_ of the ugliness inside it.
Forget about this "Hello Mr. kernel, here's what I would have done to
get what I want if I weren't such a cheap bastard, if you think about
preempting me, pretend I actually did that instead" business.  Forget
about all that RLIMIT_RTPRIO and access list stuff too, either you're
privileged or you're not, it's not like multiple users could coexist
peacefully anyway.  Maybe you could make a flavor of futex that makes
the owner non-preemptible, checks upon release or such.

Note: if you do touch futex.c, you'll definitely want to document that
you eliminated every last remote possibility of breaking anything, and
donning Nomex underwear before posting would not be a bad idea ;-)

-Mike