/*
* Select the KSE that will be run next. From that find the thread, and
* remove it from the KSEGRP's run queue. If there is thread clustering,
* this will be what does it.
*/
struct thread *
choosethread(void)
{
struct kse *ke;
struct thread *td;
struct ksegrp *kg;
#if defined(SMP) && (defined(__i386__) || defined(__amd64__))
if (smp_active == 0 && PCPU_GET(cpuid) != 0) {
/* Shutting down, run idlethread on AP's */
td = PCPU_GET(idlethread);
ke = td->td_kse;
CTR1(KTR_RUNQ, "choosethread: td=%p (idle)", td);
ke->ke_flags |= KEF_DIDRUN;
TD_SET_RUNNING(td);
return (td);
}
#endif
retry:
ke = sched_choose();
if (ke) {
td = ke->ke_thread;
KASSERT((td->td_kse == ke), ("kse/thread mismatch"));
kg = ke->ke_ksegrp;
if (td->td_proc->p_flag & P_HADTHREADS) {
if (kg->kg_last_assigned == td) {
kg->kg_last_assigned = TAILQ_PREV(td,
threadqueue, td_runq);
}
TAILQ_REMOVE(&kg->kg_runq, td, td_runq);
kg->kg_runnable--;
}
CTR2(KTR_RUNQ, "choosethread: td=%p pri=%d",
td, td->td_priority);
} else {
/* Simulate runq_choose() having returned the idle thread */
td = PCPU_GET(idlethread);
ke = td->td_kse;
CTR1(KTR_RUNQ, "choosethread: td=%p (idle)", td);
}
ke->ke_flags |= KEF_DIDRUN;
/*
* If we are in panic, only allow system threads,
* plus the one we are running in, to be run.
*/
if (panicstr && ((td->td_proc->p_flag & P_SYSTEM) == 0 &&
(td->td_flags & TDF_INPANIC) == 0)) {
/* note that it is no longer on the run queue */
TD_SET_CAN_RUN(td);
goto retry;
}
TD_SET_RUNNING(td);
return (td);
}
/*
* This function is called when a thread is about to be put on run queue
* because it has been made runnable or its priority has been adjusted. It
* determines if the new thread should be immediately preempted to. If so,
* it switches to it and eventually returns true. If not, it returns false
* so that the caller may place the thread on an appropriate run queue.
*/
int
maybe_preempt(struct thread *td)
{
#ifdef PREEMPTION
struct thread *ctd;
int cpri, pri;
/*
* The new thread should not preempt the current thread if any of the
* following conditions are true:
*
* - The kernel is in the throes of crashing (panicstr).
* - The current thread has a higher (numerically lower) or
* equivalent priority. Note that this prevents curthread from
* trying to preempt to itself.
* - It is too early in the boot for context switches (cold is set).
* - The current thread has an inhibitor set or is in the process of
* exiting. In this case, the current thread is about to switch
* out anyways, so there's no point in preempting. If we did,
* the current thread would not be properly resumed as well, so
* just avoid that whole landmine.
* - If the new thread's priority is not a realtime priority and
* the current thread's priority is not an idle priority and
* FULL_PREEMPTION is disabled.
*
* If all of these conditions are false, but the current thread is in
* a nested critical section, then we have to defer the preemption
* until we exit the critical section. Otherwise, switch immediately
* to the new thread.
*/
ctd = curthread;
THREAD_LOCK_ASSERT(td, MA_OWNED);
KASSERT((td->td_inhibitors == 0),
("maybe_preempt: trying to run inhibited thread"));
pri = td->td_priority;
cpri = ctd->td_priority;
if (panicstr != NULL || pri >= cpri || cold /* || dumping */ ||
TD_IS_INHIBITED(ctd))
return (0);
#ifndef FULL_PREEMPTION
if (pri > PRI_MAX_ITHD && cpri < PRI_MIN_IDLE)
return (0);
#endif
if (ctd->td_critnest > 1) {
CTR1(KTR_PROC, "maybe_preempt: in critical section %d",
ctd->td_critnest);
ctd->td_owepreempt = 1;
return (0);
}
/*
* Thread is runnable but not yet put on system run queue.
*/
MPASS(ctd->td_lock == td->td_lock);
MPASS(TD_ON_RUNQ(td));
TD_SET_RUNNING(td);
CTR3(KTR_PROC, "preempting to thread %p (pid %d, %s)\n", td,
td->td_proc->p_pid, td->td_name);
mi_switch(SW_INVOL | SW_PREEMPT | SWT_PREEMPT, td);
/*
* td's lock pointer may have changed. We have to return with it
* locked.
*/
spinlock_enter();
thread_unlock(ctd);
thread_lock(td);
spinlock_exit();
return (1);
#else
return (0);
#endif
}
