Re: memory barrier in ll_rw_blk.c (was Re: [PATCH][5/?] count writeback pages in nr_scanned)

From: Jens Axboe (axboe_at_suse.de)
Date: 01/06/05

  • Next message: Hirokazu Takata: "[PATCH 2.6.10-mm2] net: netconsole support for smc91x" 
    Date:	Thu, 6 Jan 2005 13:00:34 +0100
To: Nick Piggin <nickpiggin@yahoo.com.au>

    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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