> Date: Wed, 12 Jun 2019 17:04:10 +1000
> From: Jonathan Gray <j...@jsg.id.au>
>
> On Tue, Jun 11, 2019 at 09:10:46PM +0200, Mark Kettenis wrote:
> > The drm(4) codebase really needs multi-threaded task queues since the
> > code has taks that wait for the completion of other tasks that are
> > submitted to the same task queue. Thank you Linux...
> >
> > Unfortunately the code also needs to wait for the completion of
> > submitted tasks from other threads. This implemented using
> > task_barrier(9). But unfortunately our current implementation only
> > works for single-threaded task queues.
> >
> > The diff below fixes this by marking the barrier task with a flag and
> > making sure that all threads of the task queue are syncchronized.
> > This achived through a TASK_BARRIER flag that simply blocks the
> > threads until the last unblocked thread sees the flag and executes the
> > task.
> >
> > The diff also starts 4 threads for each workqueue that gets created by
> > the drm(4) layer. The number 4 is a bit arbitrary but it is the
> > number of threads that Linux creates per CPU for a so-called "unbound"
> > workqueue which hopefully is enough to always make progress.
>
> Ignoring taskletq and systq use in burner_task/switchtask across
> drivers and the local only backlight_schedule_update_status(), was it
> intentional to exclude:
>
> workqueue.h: alloc_workqueue()
> struct taskq *tq = taskq_create(name, 1, IPL_TTY, 0);
> workqueue.h: alloc_ordered_workqueue()
> struct taskq *tq = taskq_create(name, 1, IPL_TTY, 0);
> workqueue.h:
> #define system_power_efficient_wq ((struct workqueue_struct *)systq)
Not entirely. But I don't want to spawn too many threads... If this
diff seems to work, we might want to tweak the numbers a bit to see
what works best. Whatever we do, the ordered workqueues need to stay
single-threaded.
> > Please test. If you experience a "hang" with this diff, please try to
> > log in to the machine remotely over ssh and send me and jsg@ the
> > output of "ps -AHwlk".
> >
> > Thanks,
> >
> > Mark
> >
> >
> > Index: dev/pci/drm/drm_linux.c
> > ===================================================================
> > RCS file: /cvs/src/sys/dev/pci/drm/drm_linux.c,v
> > retrieving revision 1.38
> > diff -u -p -r1.38 drm_linux.c
> > --- dev/pci/drm/drm_linux.c 9 Jun 2019 12:58:30 -0000 1.38
> > +++ dev/pci/drm/drm_linux.c 11 Jun 2019 18:54:38 -0000
> > @@ -1399,15 +1399,15 @@ drm_linux_init(void)
> > {
> > if (system_wq == NULL) {
> > system_wq = (struct workqueue_struct *)
> > - taskq_create("drmwq", 1, IPL_HIGH, 0);
> > + taskq_create("drmwq", 4, IPL_HIGH, 0);
> > }
> > if (system_unbound_wq == NULL) {
> > system_unbound_wq = (struct workqueue_struct *)
> > - taskq_create("drmubwq", 1, IPL_HIGH, 0);
> > + taskq_create("drmubwq", 4, IPL_HIGH, 0);
> > }
> > if (system_long_wq == NULL) {
> > system_long_wq = (struct workqueue_struct *)
> > - taskq_create("drmlwq", 1, IPL_HIGH, 0);
> > + taskq_create("drmlwq", 4, IPL_HIGH, 0);
> > }
> >
> > if (taskletq == NULL)
> > Index: dev/pci/drm/i915/intel_hotplug.c
> > ===================================================================
> > RCS file: /cvs/src/sys/dev/pci/drm/i915/intel_hotplug.c,v
> > retrieving revision 1.2
> > diff -u -p -r1.2 intel_hotplug.c
> > --- dev/pci/drm/i915/intel_hotplug.c 14 Apr 2019 10:14:52 -0000
> > 1.2
> > +++ dev/pci/drm/i915/intel_hotplug.c 11 Jun 2019 18:54:38 -0000
> > @@ -619,7 +619,6 @@ void intel_hpd_init_work(struct drm_i915
> > INIT_WORK(&dev_priv->hotplug.hotplug_work, i915_hotplug_work_func);
> > INIT_WORK(&dev_priv->hotplug.dig_port_work, i915_digport_work_func);
> > INIT_WORK(&dev_priv->hotplug.poll_init_work, i915_hpd_poll_init_work);
> > - dev_priv->hotplug.poll_init_work.tq = systq;
> > INIT_DELAYED_WORK(&dev_priv->hotplug.reenable_work,
> > intel_hpd_irq_storm_reenable_work);
> > }
> > Index: kern/kern_task.c
> > ===================================================================
> > RCS file: /cvs/src/sys/kern/kern_task.c,v
> > retrieving revision 1.25
> > diff -u -p -r1.25 kern_task.c
> > --- kern/kern_task.c 28 Apr 2019 04:20:40 -0000 1.25
> > +++ kern/kern_task.c 11 Jun 2019 18:54:39 -0000
> > @@ -43,6 +43,7 @@ struct taskq {
> > TQ_S_DESTROYED
> > } tq_state;
> > unsigned int tq_running;
> > + unsigned int tq_barrier;
> > unsigned int tq_nthreads;
> > unsigned int tq_flags;
> > const char *tq_name;
> > @@ -59,6 +60,7 @@ static const char taskq_sys_name[] = "sy
> > struct taskq taskq_sys = {
> > TQ_S_CREATED,
> > 0,
> > + 0,
> > 1,
> > 0,
> > taskq_sys_name,
> > @@ -77,6 +79,7 @@ static const char taskq_sys_mp_name[] =
> > struct taskq taskq_sys_mp = {
> > TQ_S_CREATED,
> > 0,
> > + 0,
> > 1,
> > TASKQ_MPSAFE,
> > taskq_sys_mp_name,
> > @@ -122,6 +125,7 @@ taskq_create(const char *name, unsigned
> >
> > tq->tq_state = TQ_S_CREATED;
> > tq->tq_running = 0;
> > + tq->tq_barrier = 0;
> > tq->tq_nthreads = nthreads;
> > tq->tq_name = name;
> > tq->tq_flags = flags;
> > @@ -161,6 +165,7 @@ taskq_destroy(struct taskq *tq)
> > panic("unexpected %s tq state %u", tq->tq_name, tq->tq_state);
> > }
> >
> > + tq->tq_barrier = 0;
> > while (tq->tq_running > 0) {
> > wakeup(tq);
> > msleep(&tq->tq_running, &tq->tq_mtx, PWAIT, "tqdestroy", 0);
> > @@ -223,6 +228,7 @@ taskq_barrier(struct taskq *tq)
> >
> > WITNESS_CHECKORDER(&tq->tq_lock_object, LOP_NEWORDER, NULL);
> >
> > + SET(t.t_flags, TASK_BARRIER);
> > task_add(tq, &t);
> > cond_wait(&c, "tqbar");
> > }
> > @@ -238,6 +244,7 @@ taskq_del_barrier(struct taskq *tq, stru
> > if (task_del(tq, del))
> > return;
> >
> > + SET(t.t_flags, TASK_BARRIER);
> > task_add(tq, &t);
> > cond_wait(&c, "tqbar");
> > }
> > @@ -304,6 +311,7 @@ taskq_next_work(struct taskq *tq, struct
> > struct task *next;
> >
> > mtx_enter(&tq->tq_mtx);
> > +retry:
> > while ((next = TAILQ_FIRST(&tq->tq_worklist)) == NULL) {
> > if (tq->tq_state != TQ_S_RUNNING) {
> > mtx_leave(&tq->tq_mtx);
> > @@ -311,6 +319,17 @@ taskq_next_work(struct taskq *tq, struct
> > }
> >
> > msleep(tq, &tq->tq_mtx, PWAIT, "bored", 0);
> > + }
> > +
> > + if (ISSET(next->t_flags, TASK_BARRIER)) {
> > + if (++tq->tq_barrier == tq->tq_nthreads) {
> > + tq->tq_barrier = 0;
> > + wakeup(tq);
> > + } else {
> > + while (tq->tq_barrier > 0)
> > + msleep(tq, &tq->tq_mtx, PWAIT, "tqblk", 0);
> > + goto retry;
> > + }
> > }
> >
> > TAILQ_REMOVE(&tq->tq_worklist, next, t_entry);
> > Index: sys/task.h
> > ===================================================================
> > RCS file: /cvs/src/sys/sys/task.h,v
> > retrieving revision 1.15
> > diff -u -p -r1.15 task.h
> > --- sys/task.h 28 Apr 2019 04:20:40 -0000 1.15
> > +++ sys/task.h 11 Jun 2019 18:54:39 -0000
> > @@ -31,6 +31,7 @@ struct task {
> > };
> >
> > #define TASK_ONQUEUE 1
> > +#define TASK_BARRIER 2
> >
> > TAILQ_HEAD(task_list, task);
> >
> >
>
