On Sun, 19 Nov 2006, Oleg Nesterov wrote:
> On 11/17, Jens Axboe wrote:
> >
> > It works for me, but the overhead is still large. Before it would take
> > 8-12 jiffies for a synchronize_srcu() to complete without there actually
> > being any reader locks active, now it takes 2-3 jiffies. So it's
> > definitely faster, and as suspected the loss of two of three
> > synchronize_sched() cut down the overhead to a third.
> >
> > It's still too heavy for me, by far the most calls I do to
> > synchronize_srcu() doesn't have any reader locks pending. I'm still a
> > big advocate of the fastpath srcu_readers_active() check. I can
> > understand the reluctance to make it the default, but for my case it's
> > "safe enough", so if we could either export srcu_readers_active() or
> > export a synchronize_srcu_fast() (or something like that), then SRCU
> > would be a good fit for barrier vs plug rework.
>
> Just an idea. How about another variant of srcu which is more optimized
> for writers?
>
> struct xxx_struct {
> int completed;
> atomic_t ctr[2];
> struct mutex mutex;
> wait_queue_head_t wq;
> };
>
> void init_xxx_struct(struct xxx_struct *sp)
> {
> sp->completed = 0;
> atomic_set(sp->ctr + 0, 1);
> atomic_set(sp->ctr + 1, 1);
> mutex_init(&sp->mutex);
> init_waitqueue_head(&sp->wq);
> }
>
> int xxx_read_lock(struct xxx_struct *sp)
> {
> int idx;
>
> idx = sp->completed & 0x1;
> atomic_inc(sp->ctr + idx);
> smp_mb__after_atomic_inc();
>
> return idx;
> }
>
> void xxx_read_unlock(struct xxx_struct *sp, int idx)
> {
> if (atomic_dec_and_test(sp->ctr + idx))
> wake_up(&sp->wq);
> }
>
> void synchronize_xxx(struct xxx_struct *sp)
> {
> wait_queue_t wait;
> int idx;
>
> init_wait(&wait);
> mutex_lock(&sp->mutex);
>
> idx = sp->completed++ & 0x1;
>
> for (;;) {
> prepare_to_wait(&sp->wq, &wait, TASK_UNINTERRUPTIBLE);
>
> if (!atomic_add_unless(sp->ctr + idx, -1, 1))
> break;
>
> schedule();
> atomic_inc(sp->ctr + idx);
> }
> finish_wait(&sp->wq, &wait);
>
> mutex_unlock(&sp->mutex);
> }
>
> Very simple. Note that synchronize_xxx() is O(1), doesn't poll, and could
> be optimized further.
What happens if synchronize_xxx manages to execute inbetween
xxx_read_lock's
idx = sp->completed & 0x1;
atomic_inc(sp->ctr + idx);
statements? You see, there's no way around using synchronize_sched().
Alan Stern
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