On Sat, 2014-07-26 at 11:30 -0700, Sergey Oboguev wrote: > On Sat, Jul 26, 2014 at 1:58 AM, Mike Galbraith > <umgwanakikb...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
