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sbitmap: optimize wakeup check

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Even if we have no waiters on any of the sbitmap_queue wait states, we
still have to loop every entry to check. We do this for every IO, so
the cost adds up.

Shift a bit of the cost to the slow path, when we actually have waiters.
Wrap prepare_to_wait_exclusive() and finish_wait(), so we can maintain
an internal count of how many are currently active. Then we can simply
check this count in sbq_wake_ptr() and not have to loop if we don't
have any sleepers.

Convert the two users of sbitmap with waiting, blk-mq-tag and iSCSI.

Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe
2018-11-29 17:36:41 -07:00
parent ea86ea2cdc
commit 5d2ee7122c
4 changed files with 74 additions and 11 deletions
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12
drivers/target/iscsi/iscsi_target_util.c
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@@ -150,24 +150,26 @@ void iscsit_free_r2ts_from_list(struct iscsi_cmd *cmd)
static int iscsit_wait_for_tag(struct se_session *se_sess, int state, int *cpup)
{
int tag = -1;
DEFINE_WAIT(wait);
DEFINE_SBQ_WAIT(wait);
struct sbq_wait_state *ws;
struct sbitmap_queue *sbq;
if (state == TASK_RUNNING)
return tag;
ws = &se_sess->sess_tag_pool.ws[0];
sbq = &se_sess->sess_tag_pool;
ws = &sbq->ws[0];
for (;;) {
prepare_to_wait_exclusive(&ws->wait, &wait, state);
sbitmap_prepare_to_wait(sbq, ws, &wait, state);
if (signal_pending_state(state, current))
break;
tag = sbitmap_queue_get(&se_sess->sess_tag_pool, cpup);
tag = sbitmap_queue_get(sbq, cpup);
if (tag >= 0)
break;
schedule();
}
finish_wait(&ws->wait, &wait);
sbitmap_finish_wait(sbq, ws, &wait);
return tag;
}