Messages in this thread | | | Date | Thu, 6 Jan 2005 13:00:34 +0100 | From | Jens Axboe <> | Subject | Re: memory barrier in ll_rw_blk.c (was Re: [PATCH][5/?] count writeback pages in nr_scanned) |
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On Thu, Jan 06 2005, Nick Piggin wrote: > Jens Axboe wrote: > >On Thu, Jan 06 2005, Nick Piggin wrote: > > > > >>No that's right... but between the prepare_to_wait and the io_schedule, > >>get_request takes the lock and checks nr_requests. I think we are safe? > > > > > >It looks like it, yes you are right. But it looks to be needed a few > >lines further down instead, though :-) > > > >===== drivers/block/ll_rw_blk.c 1.281 vs edited ===== > >--- 1.281/drivers/block/ll_rw_blk.c 2004-12-01 09:13:57 +01:00 > >+++ edited/drivers/block/ll_rw_blk.c 2005-01-06 09:32:19 +01:00 > >@@ -1630,11 +1630,11 @@ > > if (rl->count[rw] < queue_congestion_off_threshold(q)) > > clear_queue_congested(q, rw); > > if (rl->count[rw]+1 <= q->nr_requests) { > >- smp_mb(); > > if (waitqueue_active(&rl->wait[rw])) > > wake_up(&rl->wait[rw]); > > blk_clear_queue_full(q, rw); > > } > >+ smp_mb(); > > if (unlikely(waitqueue_active(&rl->drain)) && > > !rl->count[READ] && !rl->count[WRITE]) > > wake_up(&rl->drain); > > > > Yes, looks like you're right there. > > Any point in doing it like this > > if (!rl->count[READ] && !rl->count[WRITE]) { > smb_mb(); > if (unlikely(waitqueue_active(...))) > wake_up() > } > > I wonder? I don't have any feeling of how memory barriers impact performance > on a very parallel system with CPUs that do lots of memory reordering like > POWER5.
I had the same idea - I think it's a good idea, it's definitely an optimization worth doing.
-- Jens Axboe
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