Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

Peter Zijlstra
2021-12-13T13:55:58Z

On Mon, Dec 06, 2021 at 12:32:22PM +0100, Peter Zijlstra wrote:
>
> Sorry, I haven't been feeling too well and as such procastinated on this
> because thinking is required :/ Trying to pick up the bits.

*sigh* and yet another week gone... someone was unhappy about refcount_t.

> No, the failure case is different; umcg_notify_resume() will simply
> block A until someone sets A::state == RUNNING and kicks it, which will
> be no-one.
>
> Now, the above situation is actually simple to fix, but it gets more
> interesting when we're using sys_umcg_wait() to build wait primitives.
> Because in that case we get stuff like:
>
> for (;;) {
> self->state = RUNNABLE;
> smp_mb();
> if (cond)
> break;
> sys_umcg_wait();
> }
> self->state = RUNNING;
>
> And we really need to not block and also not do sys_umcg_wait() early.
>
> So yes, I agree that we need a special case here that ensures
> umcg_notify_resume() doesn't block. Let me ponder naming and comments.
> Either a TF_COND_WAIT or a whole new state. I can't decide yet.
>
> Now, obviously if you do a random syscall anywhere around here, you get
> to keep the pieces :-)

Something like so I suppose..

--- a/include/uapi/linux/umcg.h
+++ b/include/uapi/linux/umcg.h
@@ -42,6 +42,32 @@
*
*/
#define UMCG_TF_PREEMPT 0x0100U
+/*
+ * UMCG_TF_COND_WAIT: indicate the task *will* call sys_umcg_wait()
+ *
+ * Enables server loops like (vs umcg_sys_exit()):
+ *
+ * for(;;) {
+ * self->status = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;
+ * // smp_mb() implied by xchg()
+ *
+ * runnable_ptr = xchg(self->runnable_workers_ptr, NULL);
+ * while (runnable_ptr) {
+ * next = runnable_ptr->runnable_workers_ptr;
+ *
+ * umcg_server_add_runnable(self, runnable_ptr);
+ *
+ * runnable_ptr = next;
+ * }
+ *
+ * self->next = umcg_server_pick_next(self);
+ * sys_umcg_wait(0, 0);
+ * }
+ *
+ * without a signal or interrupt in between setting umcg_task::state and
+ * sys_umcg_wait() resulting in an infinite wait in umcg_notify_resume().
+ */
+#define UMCG_TF_COND_WAIT 0x0200U

#define UMCG_TF_MASK 0xff00U

--- a/kernel/sched/umcg.c
+++ b/kernel/sched/umcg.c
@@ -180,7 +180,7 @@ void umcg_worker_exit(void)
/*
* Do a state transition, @from -> @to, and possible read @next after that.
*
- * Will clear UMCG_TF_PREEMPT.
+ * Will clear UMCG_TF_PREEMPT, UMCG_TF_COND_WAIT.
*
* When @to == {BLOCKED,RUNNABLE}, update timestamps.
*
@@ -216,7 +216,8 @@ static int umcg_update_state(struct task
if ((old & UMCG_TASK_MASK) != from)
goto fail;

- new = old & ~(UMCG_TASK_MASK | UMCG_TF_PREEMPT);
+ new = old & ~(UMCG_TASK_MASK |
+ UMCG_TF_PREEMPT | UMCG_TF_COND_WAIT);
new |= to & UMCG_TASK_MASK;

} while (!unsafe_try_cmpxchg_user(&self->state, &old, new, Efault));
@@ -567,11 +568,13 @@ void umcg_notify_resume(struct pt_regs *
if (state == UMCG_TASK_RUNNING)
goto done;

- // XXX can get here when:
- //
- // self->state = RUNNABLE
- // <signal>
- // sys_umcg_wait();
+ /*
+ * See comment at UMCG_TF_COND_WAIT; TL;DR: user *will* call
+ * sys_umcg_wait() and signals/interrupts shouldn't block
+ * return-to-user.
+ */
+ if (state == UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT)
+ goto done;

if (state & UMCG_TF_PREEMPT) {
if (umcg_pin_pages())
@@ -658,6 +661,13 @@ SYSCALL_DEFINE2(umcg_wait, u32, flags, u
if (ret)
goto unblock;

+ /*
+ * Clear UMCG_TF_COND_WAIT *and* check state == RUNNABLE.
+ */
+ ret = umcg_update_state(self, tsk, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNABLE);
+ if (ret)
+ goto unpin;
+
if (worker) {
ret = umcg_enqueue_runnable(tsk);
if (ret)