lkml.org 
[lkml]   [2015]   [Apr]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 1/2] sched: lockless wake-queues
From
Date
On Mon, 2015-04-20 at 14:24 -0400, George Spelvin wrote:
> +struct wake_q_head {
> + struct wake_q_node *first;
> + struct wake_q_node *last;
> +};
> +
> +#define WAKE_Q_TAIL ((struct wake_q_node *) 0x01)
> +
> +#define WAKE_Q(name) \
> + struct wake_q_head name = { WAKE_Q_TAIL, WAKE_Q_TAIL }
>
> Is there some reason you don't use the simpler singly-linked list
> construction with the tail being a pointer to a pointer:

Sure, that would also work.

>
> struct wake_q_head {
> struct wake_q_node *first, **lastp;
> };
>
> #define WAKE_Q(name) \
> struct wake_q_head name = { WAKE_Q_TAIL, &name.first }
>
>
> That removes a conditional from wake_q_add:
>
> +/*
> + * Queue a task for later wake-up by wake_up_q(). If the task is already
> + * queued by someone else, leave it to them to deliver the wakeup.

This is already commented in the cmpxchg.

> + *
> + * This property makes it impossible to guarantee the order of wakeups,
> + * but for efficiency we try to deliver wakeups in the order tasks
> + * are added.

Ok.

> If we didn't mind reversing the order, a LIFO stack
> + * would be simpler.

While true, I don't think it belongs here.

> + */
> +void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> +{
> + struct wake_q_node *node = &task->wake_q;
> +
> + /*
> + * Atomically grab the task, if ->wake_q is !nil already it means
> + * its already queued (either by us or someone else) and will get the
> + * wakeup due to that.
> + *
> + * This cmpxchg() implies a full barrier, which pairs with the write
> + * barrier implied by the wakeup in wake_up_list().
> + */
> + if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
> + return;
> +
> + get_task_struct(task);
> +
> + /*
> + * The head is context local, there can be no concurrency.
> + */
> + *head->lastp = node;
> + head->lastp = &node->next;
> +}
>
> It may also be worth commenting the fact that wake_up_q() leaves the
> struct wake_q_head in a corrupt state, so don't try to do it again.

Right, we could re-init the list once the loop is complete, yes. But it
shouldn't matter due to how we use wake-queues.

Thanks,
Davidlohr



\
 
 \ /
  Last update: 2015-04-20 22:41    [W:0.060 / U:3.044 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site