Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

Peter Zijlstra
2022-01-27T12:20:13Z

On Mon, Jan 24, 2022 at 10:46:17PM +0800, Tao Zhou wrote:
> Hi Peter,
>
> On Thu, Jan 20, 2022 at 04:55:22PM +0100, Peter Zijlstra wrote:
>
> [...]
>
> > +/* pre-schedule() */
> > +void umcg_wq_worker_sleeping(struct task_struct *tsk)
> > +{
> > + struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
> > + int ret;
> > +
> > + if (!tsk->umcg_server) {
> > + /*
> > + * Already blocked before, the pages are unpinned.
> > + */
> > + return;
> > + }
> > +
> > + /* Must not fault, mmap_sem might be held. */
> > + pagefault_disable();
> > +
> > + ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNING, UMCG_TASK_BLOCKED);
> > + if (ret == -EAGAIN) {
> > + /*
> > + * Consider:
> > + *
> > + * self->state = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;
> > + * ...
> > + * sys_umcg_wait();
> > + *
> > + * and the '...' code doing a blocking syscall/fault. This
> > + * ensures that returns with UMCG_TASK_RUNNING, which will make
>
> /UMCG_TASK_RUNNING/UMCG_TASK_RUNNABLE/

So the issue is that:

self->state = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;

<#PF>
umcg_sys_enter()
umcg_pin_user_page()
schedule()
sched_submit_work()
umcg_wq_worker_sleeping()
umcg_update_state(tsk, self, UMCG_TASK_RUNNING, UMCG_TASK_BLOCKED) // -EAGAIN
UMCG_DIE()

Which is clearly not desirable.

So this additinoal thing ensures that:

umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_BLOCKED) // 0

umcg_sys_exit()
umcg_update_state(tsk, self, UMCG_TASK_BLOCKED, UMCG_TASK_RUNNABLE);
umcg_enqueue_and_wake()

umcg_notify_resume()
umcg_wait()

// must be UMCG_TASK_RUNNING here
</#PF>

So when the pagefault finally does return, it will have:
UMCG_TASK_RUNNING.

Which will then make sys_umcg_wait() return -EAGAIN and around we go.

> > + * sys_umcg_wait() return with -EAGAIN.
> > + */
> > + ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_BLOCKED);
> > + }
> > + if (ret)
> > + UMCG_DIE_PF("state");
> > +
> > + if (umcg_wake_server(tsk))
> > + UMCG_DIE_PF("wake");
> > +
> > + pagefault_enable();
> > +
> > + /*
> > + * We're going to sleep, make sure to unpin the pages, this ensures
> > + * the pins are temporary. Also see umcg_sys_exit().
> > + */
> > + umcg_unpin_pages();
> > +}