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SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
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.

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