Re: sched_userret priority adjustment patch for sched_4bsd

From: John Baldwin (jhb_at_FreeBSD.org)
Date: 09/28/04

  • Next message: Peter Holm: "Re: scheduler (sched_4bsd) questions" 
    To: freebsd-arch@FreeBSD.org
Date: Tue, 28 Sep 2004 10:56:00 -0400

    On Monday 27 September 2004 05:28 pm, Stephan Uphoff wrote:
    > On Mon, 2004-09-27 at 14:43, John Baldwin wrote:
    > > On Monday 27 September 2004 01:20 pm, Stephan Uphoff wrote:
    > > > On Mon, 2004-09-27 at 10:16, John Baldwin wrote:
    > > > > On Saturday 25 September 2004 01:29 pm, Stephan Uphoff wrote:
    > > > > > When a thread is about to return to user space it resets its
    > > > > > priority to the user level priority.
    > > > > > However after lowering the permission its priority it needs to
    > > > > > check if its priority is still better than all other runable
    > > > > > threads. This is currently not implemented.
    > > > > > Without the check the thread can block kernel or user threads with
    > > > > > better priority until a switch is forced by by an interrupt.
    > > > > >
    > > > > > The attached patch checks the relevant runqueues and threads
    > > > > > without slots in the same ksegrp and forces a thread switch if the
    > > > > > currently running thread is no longer the best thread to run after
    > > > > > it changed its priority.
    > > > > >
    > > > > > The patch should improve interactive response under heavy load
    > > > > > somewhat. It needs a lot of testing.
    > > > >
    > > > > Perhaps the better fix is to teach the schedulers to set
    > > > > TDF_NEEDRESCHED based on on a comparison against user_pri rather than
    > > > > td_priority inside of sched_add()? Having the flag set by
    > > > > sched_add() is supposed to make this sort of check unnecessary. Even
    > > > > 4.x has the same bug I think as a process can make another process
    > > > > runnable after it's priority has been boosted by a tsleep() and
    > > > > need_resched() is only called based on a comparison of p_pri. Ah, 4.x
    > > > > doesn't have the bug because it caches the priority of curproc when
    > > > > it enters the kernel and compares against that. Thus, I think the
    > > > > correct fix is more like this:
    > > > >
    > > > > Index: sched_4bsd.c
    > > > > ===================================================================
    > > > > RCS file: /usr/cvs/src/sys/kern/sched_4bsd.c,v
    > > > > retrieving revision 1.63
    > > > > diff -u -r1.63 sched_4bsd.c
    > > > > --- sched_4bsd.c 11 Sep 2004 10:07:22 -0000 1.63
    > > > > +++ sched_4bsd.c 27 Sep 2004 14:12:03 -0000
    > > > > @@ -272,7 +272,7 @@
    > > > > {
    > > > >
    > > > > mtx_assert(&sched_lock, MA_OWNED);
    > > > > - if (td->td_priority < curthread->td_priority)
    > > > > + if (td->td_priority < curthread->td_ksegrp->kg_user_pri)
    > > > > curthread->td_flags |= TDF_NEEDRESCHED;
    > > > > }
    > > > >
    > > > > Index: sched_ule.c
    > > > > ===================================================================
    > > > > RCS file: /usr/cvs/src/sys/kern/sched_ule.c,v
    > > > > retrieving revision 1.129
    > > > > diff -u -r1.129 sched_ule.c
    > > > > --- sched_ule.c 11 Sep 2004 10:07:22 -0000 1.129
    > > > > +++ sched_ule.c 27 Sep 2004 14:13:01 -0000
    > > > > @@ -723,7 +723,7 @@
    > > > > */
    > > > > pcpu = pcpu_find(cpu);
    > > > > td = pcpu->pc_curthread;
    > > > > - if (ke->ke_thread->td_priority < td->td_priority ||
    > > > > + if (ke->ke_thread->td_priority < td->td_ksegrp->kg_user_pri
    > > > > || td == pcpu->pc_idlethread) {
    > > > > td->td_flags |= TDF_NEEDRESCHED;
    > > > > ipi_selected(1 << cpu, IPI_AST);
    > > > >
    > > > > An even better fix might be to fix td_base_pri by having it be set on
    > > > > kernel entry similar to how 4.x sets curpriority. The above fix
    > > > > should be sufficient for now, however.
    > > >
    > > > I don't think that this is enough since TDF_NEEDRESCHED is thread
    > > > specific and not cpu specific.
    > >
    > > Hmm, it is CPU specific in 4.x. It could be changed back to being a
    > > per-cpu flag easily.
    >
    > But this might not help :-(.
    > Example:
    >
    > Thread A is running in the kernel and is preempted by an interrupt
    > Thread I. Thread I wakes up thread B.
    > If I->td_ksegrp->kg_user_pri <= B->td_priority TDF_NEEDRESCHED will not
    > be set.
    > If A->td_priority < B->td_priority thread A will run once I is finished
    > serving interrupts.
    > Thread A can now leave the kernel also A->td_ksegrp->kg_user_pri >
    > B->td_priority may be true.

    If A has a priority boost from tsleep() this is intentional, however. The
    priroity boosts from tsleep() are _supposed_ to do this so as to favor
    interactive tasks. Note that if you add the code to always raise td_priority
    while in the kernel as below you may end up defeating this well-known feature
    of the 4BSD scheduler.

    > > > However the thread marked with TDF_NEEDRESCHED might not be the next
    > > > thread leaving the kernel.
    > > > ( Can't really talk about ULE since I am trying to avoid looking at
    > > > another shiny irresistible time sink this week ;-)
    > > >
    > > > I think we agree that that td_priority should be set to td_base_pri on
    > > > kernel entry. Since td_base_pri is changed by sleep and condvar
    > > > functions it should also be reset on kernel entry. (Probably from a new
    > > > ksegrp field). Condvar waits should currently non cause the base
    > > > priority to change to the current priority of the thread - otherwise
    > > > td_base_pri could get stuck at a really bad user priority.
    > > > ( td->td_base_pri might end up being worse than
    > > > td->td_ksegrp->kg_user_pri when the ksegrp priority improves)
    > >
    > > Well, I think instead that td_base_pri should be set to td_priority on
    > > kernel entry (rather than the other way around). td_priority should be
    > > unchanged just because it enters the kernel.
    >
    > I guess we disagree here.
    > There are just to many resource dependencies in the kernel that can lead
    > to priority inversion (vnode locks, disk buffer ownership, etc).
    > It would be nice to delay the priority boost until a thread acquires
    > such a resource (or even trace resource dependencies and implement
    > priority inheritance) ... but this would be a huge task.
    > Boosting the priority on kernel entry is easy and less error prone. I
    > guess we had this discussion last week and we just disagree on the
    > issue.

    Well, I think you don't understand exactly what td_base_pri is supposed to
    do. :) If you want to boost td_priority on kernel entry that is fine, but is
    completely orthogonal to this discussion. If you wanted to that then you
    might do something like this:

    kernel_entry()
    {
            if (td->td_priority > PRI_KERN_MAX)
                    sched_prio(td, PRI_KERN_MAX);
            td_base_pri = td->td_priority;
    }

    i.e., just add the boost to the kernel_entry function.

    > > I think the sleep functions could then
    > > leave td_base_pri alone. (I think setting it there is wrong because
    > > td_base_pri is not quite the same as curpriority in 4.x.) What
    > > td_base_pri is really supposed to provide, btw, is the priority that the
    > > thread should go back to once it has unlocked a mutex and had its priorty
    > > boosted while it held the mutex. Arguably it should just be using
    > > kg_user_pri for this, but then you loose priority "boosts" from tsleep(),
    > > which is why td_base_pri is set in msleep(). I guess what should happen
    > > is something more like this:
    > >
    > > kernel_entry()
    > > {
    > > KASSERT(td->td_priority == td->td_ksegrp->kg_user_pri);
    > > td->td_base_pri = td->td_priority;
    > > }
    > >
    > > msleep()
    > > {
    > > sched_prio(...);
    > > td_base_pri = td->td_priority;
    > > }
    > >
    > > The TDF_NEEDRESCHED checks should be using kg_user_pri as in my previous
    > > e-mail. Also, in sched_prio(), if our priority is ever raised
    > > (numerically, logically less important), we should set TDF_NEEDRESCHED
    > > since we may need to switch (4.x does this in maybe_needresched()).
    > > Then, TDF_NEEDRESCHED could become a per-cpu flag and have it not be
    > > cleared in mi_switch() but be cleared only in userret(). Hmm, I think
    > > all of the TDF_NEEDRESCHED handling actually beings in sched_userret()
    > > btw.
    >
    > Wouldn't this lead to unnecessary round-robin switches between threads
    > with the same priority on sched_4bsd?

    It can on 4.x then. The idea is that the occasional spurious context switch
    is cheaper than doing a lot of work on each kernel exit. 

    -- 
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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