[PATCH] block: forbid to re-enable I/O stat accounting

Jerome Marchand
2009-03-20T13:33:48Z

When we stop I/O stat accounting we stop to update the in-flight
requests counter and we need this counter to be reliable for
accounting I/O stats. Unfortunately updating in_flight field may
affect performance. So, until we have a better solution, just forbid
to re-enable I/O stat accounting after it has been disabled.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
---
block/blk-sysfs.c | 13 ++++++++-----
1 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index e29ddfc..2903874 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -208,12 +208,15 @@ static ssize_t queue_iostats_store(struct request_queue *q, const char *page,
unsigned long stats;
ssize_t ret = queue_var_store(&stats, page, count);

- spin_lock_irq(q->queue_lock);
- if (stats)
- queue_flag_set(QUEUE_FLAG_IO_STAT, q);
- else
+ /*
+ * When we disable io stat accounting we stop tracking essential data,
+ * so don't allow to reenable it as it would lead to inconsistency.
+ */
+ if (!stats) {
+ spin_lock_irq(q->queue_lock);
queue_flag_clear(QUEUE_FLAG_IO_STAT, q);
- spin_unlock_irq(q->queue_lock);
+ spin_unlock_irq(q->queue_lock);
+ }

return ret;
}
--
1.5.5.1