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

[Alan Stern]

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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