Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

[Oleg Nesterov <oleg@tv-sign.ru>]

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.

Oleg.