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

[Tao Zhou <tao.zhou@linux.dev>]

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/

> +		 * 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();
> +}

[...]

> +/* Called from syscall exit path and exceptions that can schedule */
> +void umcg_sys_exit(struct pt_regs *regs)
> +{
> +	struct task_struct *tsk = current;
> +	long syscall = syscall_get_nr(tsk, regs);
> +
> +	if (syscall == __NR_umcg_wait ||
> +	    syscall == __NR_umcg_ctl)
> +		return;
> +
> +	if (tsk->umcg_server) {
> +		/*
> +		 * Didn't block, we done.
> +		 */
> +		umcg_unpin_pages();
> +		return;
> +	}
> +
> +	umcg_unblock_and_wait();

umcg_unblock_and_wait() -> umcg_enqueue_and_wake() ->
umcg_wake_server() -> umcg_wake_task(tsk->umcg_server, ...)

tsk->umcg_server is NULL here and umcg_wake_task() use it to update
state in umcg_update_state(NULL, ...), that means tsk->umcg_clock
will happen something i do not know.

There are two places to call umcg_unblock_and_wait(). One is in 
umcg_register() where the server is set. Another one is in
umcg_sys_exit() where the server is not set. May use a bool to
indicate if the server is set.

> +}

[...]

> +/**
> + * sys_umcg_wait: transfer running context
> + *
> + * Called like:
> + *
> + *	self->state = UMCG_TASK_RUNNABLE | UMCG_TF_COND_WAIT;
> + *	...
> + *	sys_umcg_wait(0, time);
> + *
> + * The syscall will clear TF_COND_WAIT and wait until state becomes RUNNING.
> + * The code '...' must not contain syscalls
> + *
> + * If self->next_tid is set and indicates a valid UMCG task with RUNNABLE state
> + * that task will be made RUNNING and woken -- transfering the running context
> + * to that task. In this case self->next_tid is modified with TID_RUNNING to
> + * indicate self->next_tid is consumed.
> + *
> + * If self->next has TID_RUNNING set, it is validated the related task has

/self->next/self->next_tid/
Things are not clear to me even they are clear now. Nice.

Thanks,
Tao