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

Tao Zhou
2022-01-24T14:46:30Z

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