/*
* Return nonzero if there are no threads sleeping on the channel.
* This is meant to be used only for diagnostic purposes.
*/
bool
wchan_isempty(struct wchan* wc, struct spinlock* lk) {
bool ret;
assert(spinlock_held(lk));
ret = threadlist_isempty(&wc->wc_threads);
return ret;
}
/*
* Return nonzero if there are no threads sleeping on the channel.
* This is meant to be used only for diagnostic purposes.
*/
bool
wchan_isempty(struct wchan *wc, struct spinlock *lk)
{
bool ret;
KASSERT(spinlock_do_i_hold(lk));
ret = threadlist_isempty(&wc->wc_threads);
return ret;
}
void
threadlist_cleanup(struct threadlist *tl)
{
DEBUGASSERT(tl != NULL);
DEBUGASSERT(tl->tl_head.tln_next == &tl->tl_tail);
DEBUGASSERT(tl->tl_head.tln_prev == NULL);
DEBUGASSERT(tl->tl_tail.tln_next == NULL);
DEBUGASSERT(tl->tl_tail.tln_prev == &tl->tl_head);
DEBUGASSERT(tl->tl_head.tln_self == NULL);
DEBUGASSERT(tl->tl_tail.tln_self == NULL);
KASSERT(threadlist_isempty(tl));
KASSERT(tl->tl_count == 0);
/* nothing (else) to do */
}
/*
* Thread migration.
*
* This is also called periodically from hardclock(). If the current
* CPU is busy and other CPUs are idle, or less busy, it should move
* threads across to those other other CPUs.
*
* Migrating threads isn't free because of cache affinity; a thread's
* working cache set will end up having to be moved to the other CPU,
* which is fairly slow. The tradeoff between this performance loss
* and the performance loss due to underutilization of some CPUs is
* something that needs to be tuned and probably is workload-specific.
*
* For here and now, because we know we're running on System/161 and
* System/161 does not (yet) model such cache effects, we'll be very
* aggressive.
*/
void
thread_consider_migration(void)
{
unsigned my_count, total_count, one_share, to_send;
unsigned i, numcpus;
struct cpu *c;
struct threadlist victims;
struct thread *t;
my_count = total_count = 0;
numcpus = cpuarray_num(&allcpus);
for (i=0; i<numcpus; i++) {
c = cpuarray_get(&allcpus, i);
spinlock_acquire(&c->c_runqueue_lock);
total_count += c->c_runqueue.tl_count;
if (c == curcpu->c_self) {
my_count = c->c_runqueue.tl_count;
}
spinlock_release(&c->c_runqueue_lock);
}
one_share = DIVROUNDUP(total_count, numcpus);
if (my_count < one_share) {
return;
}
to_send = my_count - one_share;
threadlist_init(&victims);
spinlock_acquire(&curcpu->c_runqueue_lock);
for (i=0; i<to_send; i++) {
t = threadlist_remtail(&curcpu->c_runqueue);
threadlist_addhead(&victims, t);
}
spinlock_release(&curcpu->c_runqueue_lock);
for (i=0; i < numcpus && to_send > 0; i++) {
c = cpuarray_get(&allcpus, i);
if (c == curcpu->c_self) {
continue;
}
spinlock_acquire(&c->c_runqueue_lock);
while (c->c_runqueue.tl_count < one_share && to_send > 0) {
t = threadlist_remhead(&victims);
/*
* Ordinarily, curthread will not appear on
* the run queue. However, it can under the
* following circumstances:
* - it went to sleep;
* - the processor became idle, so it
* remained curthread;
* - it was reawakened, so it was put on the
* run queue;
* - and the processor hasn't fully unidled
* yet, so all these things are still true.
*
* If the timer interrupt happens at (almost)
* exactly the proper moment, we can come here
* while things are in this state and see
* curthread. However, *migrating* curthread
* can cause bad things to happen (Exercise:
* Why? And what?) so shuffle it to the end of
* the list and decrement to_send in order to
* skip it. Then it goes back on our own run
* queue below.
*/
if (t == curthread) {
threadlist_addtail(&victims, t);
to_send--;
continue;
}
t->t_cpu = c;
threadlist_addtail(&c->c_runqueue, t);
DEBUG(DB_THREADS,
"Migrated thread %s: cpu %u -> %u",
t->t_name, curcpu->c_number, c->c_number);
to_send--;
if (c->c_isidle) {
/*
* Other processor is idle; send
* interrupt to make sure it unidles.
*/
ipi_send(c, IPI_UNIDLE);
}
}
spinlock_release(&c->c_runqueue_lock);
}
/*
* Because the code above isn't atomic, the thread counts may have
* changed while we were working and we may end up with leftovers.
* Don't panic; just put them back on our own run queue.
*/
if (!threadlist_isempty(&victims)) {
spinlock_acquire(&curcpu->c_runqueue_lock);
while ((t = threadlist_remhead(&victims)) != NULL) {
threadlist_addtail(&curcpu->c_runqueue, t);
}
spinlock_release(&curcpu->c_runqueue_lock);
}
KASSERT(threadlist_isempty(&victims));
threadlist_cleanup(&victims);
}
/*
* High level, machine-independent context switch code.
*
* The current thread is queued appropriately and its state is changed
* to NEWSTATE; another thread to run is selected and switched to.
*
* If NEWSTATE is S_SLEEP, the thread is queued on the wait channel
* WC, protected by the spinlock LK. Otherwise WC and Lk should be
* NULL.
*/
static
void
thread_switch(threadstate_t newstate, struct wchan *wc, struct spinlock *lk)
{
struct thread *cur, *next;
int spl;
DEBUGASSERT(curcpu->c_curthread == curthread);
DEBUGASSERT(curthread->t_cpu == curcpu->c_self);
/* Explicitly disable interrupts on this processor */
spl = splhigh();
cur = curthread;
/*
* If we're idle, return without doing anything. This happens
* when the timer interrupt interrupts the idle loop.
*/
if (curcpu->c_isidle) {
splx(spl);
return;
}
/* Check the stack guard band. */
thread_checkstack(cur);
/* Lock the run queue. */
spinlock_acquire(&curcpu->c_runqueue_lock);
/* Micro-optimization: if nothing to do, just return */
if (newstate == S_READY && threadlist_isempty(&curcpu->c_runqueue)) {
spinlock_release(&curcpu->c_runqueue_lock);
splx(spl);
return;
}
/* Put the thread in the right place. */
switch (newstate) {
case S_RUN:
panic("Illegal S_RUN in thread_switch\n");
case S_READY:
thread_make_runnable(cur, true /*have lock*/);
break;
case S_SLEEP:
cur->t_wchan_name = wc->wc_name;
/*
* Add the thread to the list in the wait channel, and
* unlock same. To avoid a race with someone else
* calling wchan_wake*, we must keep the wchan's
* associated spinlock locked from the point the
* caller of wchan_sleep locked it until the thread is
* on the list.
*/
threadlist_addtail(&wc->wc_threads, cur);
spinlock_release(lk);
break;
case S_ZOMBIE:
cur->t_wchan_name = "ZOMBIE";
threadlist_addtail(&curcpu->c_zombies, cur);
break;
}
cur->t_state = newstate;
/*
* Get the next thread. While there isn't one, call md_idle().
* curcpu->c_isidle must be true when md_idle is
* called. Unlock the runqueue while idling too, to make sure
* things can be added to it.
*
* Note that we don't need to unlock the runqueue atomically
* with idling; becoming unidle requires receiving an
* interrupt (either a hardware interrupt or an interprocessor
* interrupt from another cpu posting a wakeup) and idling
* *is* atomic with respect to re-enabling interrupts.
*
* Note that c_isidle becomes true briefly even if we don't go
* idle. However, because one is supposed to hold the runqueue
* lock to look at it, this should not be visible or matter.
*/
/* The current cpu is now idle. */
curcpu->c_isidle = true;
do {
next = threadlist_remhead(&curcpu->c_runqueue);
if (next == NULL) {
spinlock_release(&curcpu->c_runqueue_lock);
cpu_idle();
spinlock_acquire(&curcpu->c_runqueue_lock);
}
} while (next == NULL);
curcpu->c_isidle = false;
/*
* Note that curcpu->c_curthread may be the same variable as
* curthread and it may not be, depending on how curthread and
* curcpu are defined by the MD code. We'll assign both and
* assume the compiler will optimize one away if they're the
* same.
*/
curcpu->c_curthread = next;
curthread = next;
/* do the switch (in assembler in switch.S) */
switchframe_switch(&cur->t_context, &next->t_context);
/*
* When we get to this point we are either running in the next
* thread, or have come back to the same thread again,
* depending on how you look at it. That is,
* switchframe_switch returns immediately in another thread
* context, which in general will be executing here with a
* different stack and different values in the local
* variables. (Although new threads go to thread_startup
* instead.) But, later on when the processor, or some
* processor, comes back to the previous thread, it's also
* executing here with the *same* value in the local
* variables.
*
* The upshot, however, is as follows:
*
* - The thread now currently running is "cur", not "next",
* because when we return from switchrame_switch on the
* same stack, we're back to the thread that
* switchframe_switch call switched away from, which is
* "cur".
*
* - "cur" is _not_ the thread that just *called*
* switchframe_switch.
*
* - If newstate is S_ZOMB we never get back here in that
* context at all.
*
* - If the thread just chosen to run ("next") was a new
* thread, we don't get to this code again until
* *another* context switch happens, because when new
* threads return from switchframe_switch they teleport
* to thread_startup.
*
* - At this point the thread whose stack we're now on may
* have been migrated to another cpu since it last ran.
*
* The above is inherently confusing and will probably take a
* while to get used to.
*
* However, the important part is that code placed here, after
* the call to switchframe_switch, does not necessarily run on
* every context switch. Thus any such code must be either
* skippable on some switches or also called from
* thread_startup.
*/
/* Clear the wait channel and set the thread state. */
cur->t_wchan_name = NULL;
cur->t_state = S_RUN;
/* Unlock the run queue. */
spinlock_release(&curcpu->c_runqueue_lock);
/* Activate our address space in the MMU. */
as_activate();
/* Clean up dead threads. */
exorcise();
/* Turn interrupts back on. */
splx(spl);
}
