Date: Fri, 10 Dec 2010 21:56:58 -0800 From: Julian Elischer <julian@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: Kostik Belousov <kostikbel@gmail.com>, arch@freebsd.org Subject: Re: Realtime thread priorities Message-ID: <4D0312AA.7010009@freebsd.org> In-Reply-To: <201012101641.51652.jhb@freebsd.org> References: <201012101050.45214.jhb@freebsd.org> <201012101133.55389.jhb@freebsd.org> <20101210195716.GE33073@deviant.kiev.zoral.com.ua> <201012101641.51652.jhb@freebsd.org>
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On 12/10/10 1:41 PM, John Baldwin wrote:
> On Friday, December 10, 2010 2:57:16 pm Kostik Belousov wrote:
>> On Fri, Dec 10, 2010 at 11:33:55AM -0500, John Baldwin wrote:
>>> 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).
If you think how RT scheduling works when they stick an RT shim under
an OS
then it becomes obvious that all RT threads trump all TS threads,
kernel or not.
basically they have a separate RT scheduler that gets to schedule all
RT threads and
they only even bother to run the NON RT (TS) scheduler when there is
spare time.
TS threads are only ever scheduled by the RT scheduler when they own some
resource needed by an RT thread.
>>>> 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.
the aim of RT is to be unfair.
(to TS threads)
>>> Err, I think that what you describe is the current case and is what #2 would
>>> seek to change.
>> Sorry, might be my language was not clear, but I said "Would not doing
>> #2 allow ...", i.e. I specifically mean that we shall do #2 to avoid the
>> situation I described.
> Ah, yes, it does allow that. As bde@ said though, the overhead of extra
> context switches in the common case might not be worth it.
>
> I have a possible patch for 1), but it involves fixing a few places and is
> only compile tested yet (will run test it soon). I also think that in my
> case I almost always want 1) anyway (my realtime processes are always more
> important than sshd, even while sshd is in the kernel):
>
> Index: kern/kern_synch.c
> ===================================================================
> --- kern/kern_synch.c (revision 215592)
> +++ kern/kern_synch.c (working copy)
> @@ -214,7 +214,8 @@
> * Adjust this thread's priority, if necessary.
> */
> pri = priority& PRIMASK;
> - if (pri != 0&& pri != td->td_priority) {
> + if (pri != 0&& pri != td->td_priority&&
> + td->td_pri_class == PRI_TIMESHARE) {
> thread_lock(td);
> sched_prio(td, pri);
> thread_unlock(td);
> @@ -552,7 +553,8 @@
> {
>
> thread_lock(td);
> - sched_prio(td, PRI_MAX_TIMESHARE);
> + if (td->td_pri_class == PRI_TIMESHARE)
> + sched_prio(td, PRI_MAX_TIMESHARE);
> mi_switch(SW_VOL, NULL);
> thread_unlock(td);
> td->td_retval[0] = 0;
> Index: kern/subr_sleepqueue.c
> ===================================================================
> --- kern/subr_sleepqueue.c (revision 215592)
> +++ kern/subr_sleepqueue.c (working copy)
> @@ -693,7 +720,8 @@
>
> /* Adjust priority if requested. */
> MPASS(pri == -1 || (pri>= PRI_MIN&& pri<= PRI_MAX));
> - if (pri != -1&& td->td_priority> pri)
> + if (pri != -1&& td->td_priority> pri&&
> + td->td_pri_class == PRI_TIMESHARE)
> sched_prio(td, pri);
> return (setrunnable(td));
> }
> Index: sys/priority.h
> ===================================================================
> --- sys/priority.h (revision 215592)
> +++ sys/priority.h (working copy)
> @@ -68,8 +68,8 @@
> * are insignificant. Ranges are as follows:
> *
> * Interrupt threads: 0 - 63
> - * Top half kernel threads: 64 - 127
> - * Realtime user threads: 128 - 159
> + * Realtime user threads: 64 - 95
> + * Top half kernel threads: 96 - 159
> * Time sharing user threads: 160 - 223
> * Idle user threads: 224 - 255
> *
> @@ -81,7 +81,7 @@
> #define PRI_MAX (255) /* Lowest priority. */
>
> #define PRI_MIN_ITHD (PRI_MIN)
> -#define PRI_MAX_ITHD (PRI_MIN_KERN - 1)
> +#define PRI_MAX_ITHD (PRI_MIN_REALTIME - 1)
>
> #define PI_REALTIME (PRI_MIN_ITHD + 0)
> #define PI_AV (PRI_MIN_ITHD + 4)
> @@ -94,9 +94,12 @@
> #define PI_DULL (PRI_MIN_ITHD + 32)
> #define PI_SOFT (PRI_MIN_ITHD + 36)
>
> -#define PRI_MIN_KERN (64)
> -#define PRI_MAX_KERN (PRI_MIN_REALTIME - 1)
> +#define PRI_MIN_REALTIME (64)
> +#define PRI_MAX_REALTIME (PRI_MIN_KERN - 1)
>
> +#define PRI_MIN_KERN (96)
> +#define PRI_MAX_KERN (PRI_MIN_TIMESHARE - 1)
> +
> #define PSWP (PRI_MIN_KERN + 0)
> #define PVM (PRI_MIN_KERN + 4)
> #define PINOD (PRI_MIN_KERN + 8)
> @@ -109,9 +112,6 @@
> #define PLOCK (PRI_MIN_KERN + 36)
> #define PPAUSE (PRI_MIN_KERN + 40)
>
> -#define PRI_MIN_REALTIME (128)
> -#define PRI_MAX_REALTIME (PRI_MIN_TIMESHARE - 1)
> -
> #define PRI_MIN_TIMESHARE (160)
> #define PRI_MAX_TIMESHARE (PRI_MIN_IDLE - 1)
>
>
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