Messages in this thread | | | Subject | Re: [PATCH 1/2] sched: lockless wake-queues | From | Davidlohr Bueso <> | Date | Mon, 20 Apr 2015 13:08:20 -0700 |
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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
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