Re: sched_userret priority adjustment patch for sched_4bsd

From: John Baldwin (jhb_at_FreeBSD.org)
Date: 09/27/04

  • Next message: Stephan Uphoff: "Re: sched_userret priority adjustment patch for sched_4bsd" 
    To: freebsd-arch@FreeBSD.org
Date: Mon, 27 Sep 2004 10:16:13 -0400

    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. 

    -- 
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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