Date: Fri, 10 Dec 2010 11:33:55 -0500 From: John Baldwin <jhb@freebsd.org> To: Kostik Belousov <kostikbel@gmail.com> Cc: arch@freebsd.org Subject: Re: Realtime thread priorities Message-ID: <201012101133.55389.jhb@freebsd.org> In-Reply-To: <20101210162631.GC33073@deviant.kiev.zoral.com.ua> References: <201012101050.45214.jhb@freebsd.org> <20101210162631.GC33073@deviant.kiev.zoral.com.ua>
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On Friday, December 10, 2010 11:26:31 am Kostik Belousov wrote: > On Fri, Dec 10, 2010 at 10:50:45AM -0500, John Baldwin wrote: > > So I finally had a case today where I wanted to use rtprio but it doesn't seem > > very useful in its current state. Specifically, I want to be able to tag > > certain user processes as being more important than any other user processes > > even to the point that if one of my important processes blocks on a mutex, the > > owner of that mutex should be more important than sshd being woken up from > > sbwait by new data (for example). This doesn't work currently with rtprio due > > to the way the priorities are laid out (and I believe I probably argued for > > the current layout back when it was proposed). > > > > The current layout breaks up the global thread priority space (0 - 255) into a > > couple of bands: > > > > 0 - 63 : interrupt threads > > 64 - 127 : kernel sleep priorities (PSOCK, etc.) > > 128 - 159 : real-time user threads (rtprio) > > 160 - 223 : time-sharing user threads > > 224 - 255 : idle threads (idprio and kernel idle procs) > > > > The problem I am running into is that when a time-sharing thread goes to sleep > > in the kernel (waiting on select, socket data, tty, etc.) it actually ends up > > in the kernel priorities range (64 - 127). This means when it wakes up it > > will trump (and preempt) a real-time user thread even though these processes > > nominally have a priority down in the 160 - 223 range. We do drop the kernel > > sleep priority during userret(), but we don't recheck the scheduler queues to > > see if we should preempt the thread during userret(), so it effectively runs > > with the kernel sleep priority for the rest of the quantum while it is in > > userland. > > > > My first question is if this behavior is the desired behavior? Originally I > > think I preferred the current layout because I thought a thread in the kernel > > should always have priority so it can release locks, etc. However, priority > > propagation should actually handle the case of some very important thread > > needing a lock. In my use case today where I actually want to use rtprio I > > think I want different behavior where the rtprio thread is more important than > > the thread waking up with PSOCK, etc. > > > > If we decide to change the behavior I see two possible fixes: > > > > 1) (easy) just move the real-time priority range above the kernel sleep > > priority range > > > > 2) (harder) make sched_userret() check the run queue to see if it should > > preempt when dropping the kernel sleep priority. I think bde@ has suggested > > that we should do this for correctness previously (and I've had some old, > > unfinished patches to do this in a branch in p4 for several years). > > Would not doing #2 allow e.g. two threads that perform ping-pong with > a single byte read/write into a socket to usurp the CPU ? The threads > could try to also do some CPU-intensive calculations for some time > during the quantum too. > > Such threads are arguably "interactive", but I think that the gain is > priority is too unfair. Err, I think that what you describe is the current case and is what #2 would seek to change. -- John Baldwin
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