Messages in this thread | | | Date | Sun, 19 Nov 2006 22:00:27 +0300 | From | Oleg Nesterov <> | Subject | Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync |
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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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