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SubjectRe: [RFC][PATCH 3/3] sched: User Mode Concurency Groups
On Fri, Jan 14, 2022 at 03:09:55PM +0100, Peter Zijlstra wrote:
> > > +SYSCALL_DEFINE3(umcg_ctl, u32, flags, struct umcg_task __user *, self, clockid_t, which_clock)
> > > +{
> > > + struct task_struct *server;
> > > + struct umcg_task ut;
> > > +
> > > + if ((unsigned long)self % UMCG_TASK_ALIGN)
> > > + return -EINVAL;
> > > +
> > > + if (flags & ~(UMCG_CTL_REGISTER |
> > > + UMCG_CTL_UNREGISTER |
> > > + UMCG_CTL_WORKER))
> > > + return -EINVAL;
> > > +
> > > + if (flags == UMCG_CTL_UNREGISTER) {
> > > + if (self || !current->umcg_task)
> > > + return -EINVAL;
> > > +
> > > + if (current->flags & PF_UMCG_WORKER)
> > > + umcg_worker_exit();
> >
> > The server should be woken here. Imagine: one server, one worker.
> > The server is sleeping, the worker is running. The worker unregisters,
> > the server keeps sleeping forever?
> >
> > I'm OK re: NOT waking the server if the worker thread exits without
> > unregistering, as this is the userspace breaking the contract/protocol.
> > But here we do need to notify the server. At the minimum so that the
> > server can schedule a worker to run in its place.
> >
> > (Why is this important? Worker count can fluctuate considerably:
> > on load spikes many new workers may be created, and later in
> > quiet times they exit to free resources.)
>
> Fair enough. Will do.

Something like so then...

---
--- a/kernel/sched/umcg.c
+++ b/kernel/sched/umcg.c
@@ -185,13 +185,6 @@ void umcg_clear_child(struct task_struct
umcg_clear_task(tsk);
}

-/* Called both by normally (unregister) and abnormally exiting workers. */
-void umcg_worker_exit(void)
-{
- umcg_unpin_pages();
- umcg_clear_task(current);
-}
-
/*
* Do a state transition: @from -> @to.
*
@@ -748,32 +741,43 @@ SYSCALL_DEFINE2(umcg_wait, u32, flags, u
* sys_umcg_ctl: (un)register the current task as a UMCG task.
* @flags: ORed values from enum umcg_ctl_flag; see below;
* @self: a pointer to struct umcg_task that describes this
- * task and governs the behavior of sys_umcg_wait if
- * registering; must be NULL if unregistering.
+ * task and governs the behavior of sys_umcg_wait.
* @which_clock: clockid to use for timestamps and timeouts
*
* @flags & UMCG_CTL_REGISTER: register a UMCG task:
*
- * UMCG workers:
- * - @flags & UMCG_CTL_WORKER
- * - self->state must be UMCG_TASK_BLOCKED
- *
- * UMCG servers:
- * - !(@flags & UMCG_CTL_WORKER)
- * - self->state must be UMCG_TASK_RUNNING
- *
- * All tasks:
- * - self->server_tid must be a valid server
- * - self->next_tid must be zero
- *
- * If the conditions above are met, sys_umcg_ctl() immediately returns
- * if the registered task is a server. If the registered task is a
- * worker it will be added to it's server's runnable_workers_ptr list
- * and the server will be woken.
- *
- * @flags == UMCG_CTL_UNREGISTER: unregister a UMCG task. If the current task
- * is a UMCG worker, the userspace is responsible for waking its
- * server (before or after calling sys_umcg_ctl).
+ * UMCG workers:
+ * - @flags & UMCG_CTL_WORKER
+ * - self->state must be UMCG_TASK_BLOCKED
+ *
+ * UMCG servers:
+ * - !(@flags & UMCG_CTL_WORKER)
+ * - self->state must be UMCG_TASK_RUNNING
+ *
+ * All tasks:
+ * - self->server_tid must be a valid server
+ * - self->next_tid must be zero
+ *
+ * If the conditions above are met, sys_umcg_ctl() immediately returns
+ * if the registered task is a server. If the registered task is a
+ * worker it will be added to it's server's runnable_workers_ptr list
+ * and the server will be woken.
+ *
+ * @flags & UMCG_CTL_UNREGISTER: unregister a UMCG task.
+ *
+ * UMCG workers:
+ * - @flags & UMCG_CTL_WORKER
+ *
+ * UMCG servers:
+ * - !(@flags & UMCG_CTL_WORKER)
+ *
+ * All tasks:
+ * - self must match with UMCG_CTL_REGISTER
+ * - self->state must be UMCG_TASK_RUNNING
+ * - self->server_tid must be a valid server
+ *
+ * If the conditions above are met, sys_umcg_ctl() will change state to
+ * UMCG_TASK_NONE, and for workers, wake either next or server.
*
* Return:
* 0 - success
@@ -794,16 +798,31 @@ SYSCALL_DEFINE3(umcg_ctl, u32, flags, st
UMCG_CTL_WORKER))
return -EINVAL;

- if (flags == UMCG_CTL_UNREGISTER) {
- if (self || !current->umcg_task)
+ if (flags & UMCG_CTL_UNREGISTER) {
+ int ret;
+
+ if (!self || self != current->umcg_task)
return -EINVAL;

- if (current->flags & PF_UMCG_WORKER) {
- umcg_worker_exit();
- // XXX wake server
- } else
- umcg_clear_task(current);
+ current->flags &= ~PF_UMCG_WORKER;

+ ret = umcg_pin_pages();
+ if (ret) {
+ current->flags |= PF_UMCG_WORKER;
+ return ret;
+ }
+
+ ret = umcg_update_state(current, self, UMCG_TASK_RUNNING, UMCG_TASK_NONE);
+ if (ret) {
+ current->flags |= PF_UMCG_WORKER;
+ return ret;
+ }
+
+ if (current->flags & PF_UMCG_WORKER)
+ umcg_wake(current);
+
+ umcg_unpin_pages();
+ umcg_clear_task(current);
return 0;
}

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