Date: Mon, 27 Sep 2004 10:16:13 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-arch@FreeBSD.org Cc: Stephan Uphoff <ups@tree.com> Subject: Re: sched_userret priority adjustment patch for sched_4bsd Message-ID: <200409271016.13345.jhb@FreeBSD.org> In-Reply-To: <1096133353.53798.17613.camel@palm.tree.com> References: <1096133353.53798.17613.camel@palm.tree.com>
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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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