[lkml]   [2006]   [Nov]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
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);

int xxx_read_lock(struct xxx_struct *sp)
int idx;

idx = sp->completed & 0x1;
atomic_inc(sp->ctr + idx);

return idx;

void xxx_read_unlock(struct xxx_struct *sp, int idx)
if (atomic_dec_and_test(sp->ctr + idx))

void synchronize_xxx(struct xxx_struct *sp)
wait_queue_t wait;
int idx;


idx = sp->completed++ & 0x1;

for (;;) {
prepare_to_wait(&sp->wq, &wait, TASK_UNINTERRUPTIBLE);

if (!atomic_add_unless(sp->ctr + idx, -1, 1))

atomic_inc(sp->ctr + idx);
finish_wait(&sp->wq, &wait);


Very simple. Note that synchronize_xxx() is O(1), doesn't poll, and could
be optimized further.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2006-11-19 19:01    [W:0.188 / U:1.608 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site