Re: [PATCH -next RFC v2 8/8] sbitmap: wake up the number of threads based on required tags

[yukuai (C)]

在 2022/04/09 5:13, Bart Van Assche 写道:
> On 4/8/22 00:39, Yu Kuai wrote:
> > +static unsigned int get_wake_nr(struct sbq_wait_state *ws, unsigned 
> > int nr_tags)
> > +{
> > +    struct sbq_wait *wait;
> > +    struct wait_queue_entry *entry;
> > +    unsigned int nr = 1;
> > +
> > +    spin_lock_irq(&ws->wait.lock);
> > +    list_for_each_entry(entry, &ws->wait.head, entry) {
> > +        wait = container_of(entry, struct sbq_wait, wait);
> > +        if (nr_tags <= wait->nr_tags)
> > +            break;
> > +
> > +        nr++;
> > +        nr_tags -= wait->nr_tags;
> > +    }
> > +    spin_unlock_irq(&ws->wait.lock);
> > +
> > +    return nr;
> > +}
> > +
> >   static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> >   {
> >       struct sbq_wait_state *ws;
> > @@ -648,7 +668,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> >       smp_mb__before_atomic();
> >       atomic_set(&ws->wait_cnt, wake_batch);
> >       sbq_update_preemption(sbq, wake_batch);
> > -    wake_up_nr(&ws->wait, wake_batch);
> > +    wake_up_nr(&ws->wait, get_wake_nr(ws, wake_batch));
> >       return true;
> >   }
>
> An additional comment: my understanding is that __sbq_wake_up() should 
> wake up exactly `wake_batch` waiters. The above patch changes that into 
> waking up at most `wake_batch` waiters. I think that's wrong.
Hi,

I think the reason to wake up 'wake_batch' waiters is to make sure
wakers will use up 'wake_batch' tags that is just freed, because each
wakers should aquire at least one tag. Thus I think if we can make sure
wakers will use up 'wake_batch' tags, it's ok to wake up less waiters.

Please kindly correct me if I'm wrong.

Thanks,
Kuai
> Bart.
> .