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Subject[PATCH v3] blk: fix a wrong accounting of hd_struct->in_flight
Hi Jens,

Jens Axboe wrote:
> On 2010-10-14 14:48, Yasuaki Ishimatsu wrote:
>> Index: linux-2.6.36-rc7/block/blk-core.c
>> ===================================================================
>> --- linux-2.6.36-rc7.orig/block/blk-core.c 2010-10-07 05:39:52.000000000 +0900
>> +++ linux-2.6.36-rc7/block/blk-core.c 2010-10-14 17:25:43.000000000 +0900
>> @@ -66,9 +66,15 @@ static void drive_stat_acct(struct reque
>> cpu = part_stat_lock();
>> part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
>>
>> - if (!new_io)
>> + if (!new_io) {
>> + if (unlikely(rq->part != part)) {
>> + part_dec_in_flight(rq->part, rw);
>> + part_inc_in_flight(part, rw);
>> + rq->part = part;
>> + }
>> part_stat_inc(cpu, part, merges[rw]);
>> - else {
>> + } else {
>> + rq->part = part;
>> part_round_stats(cpu, part);
>> part_inc_in_flight(part, rw);
>> }
>
> I was thinking that we'd do away with the lookup always if ->part was
> already set. It will probably require a quiscing of IO on partition
> table reload, though.

O.K.
I removed extra part lookups. Following patch also fixed a wrong accounting of
hd_struct->in_flight. But I could not invent how to stop IOs when
reloading partition table. Do you have some idea?

Thsanks,
Yasuaki Ishimatsu
===

From: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>

/proc/diskstats would display a strange output as follows.

$ cat /proc/diskstats |grep sda
8 0 sda 90524 7579 102154 20464 0 0 0 0 0 14096 20089
8 1 sda1 19085 1352 21841 4209 0 0 0 0 4294967064 15689 4293424691
~~~~~~~~~~
8 2 sda2 71252 3624 74891 15950 0 0 0 0 232 23995 1562390
8 3 sda3 54 487 2188 92 0 0 0 0 0 88 92
8 4 sda4 4 0 8 0 0 0 0 0 0 0 0
8 5 sda5 81 2027 2130 138 0 0 0 0 0 87 137
Its reason is the wrong way of accounting hd_struct->in_flight. When a bio is
merged into a request belongs to different partition by ELEVATOR_FRONT_MERGE.

The detailed root cause is as follows.

Assuming that there are two partition, sda1 and sda2.

1. A request for sda2 is in request_queue. Hence sda1's hd_struct->in_flight
is 0 and sda2's one is 1.

| hd_struct->in_flight
---------------------------
sda1 | 0
sda2 | 1
---------------------------
2. A bio belongs to sda1 is issued and is merged into the request mentioned on
step1 by ELEVATOR_BACK_MERGE. The first sector of the request is changed
from sda2 region to sda1 region. However the two partition's
hd_struct->in_flight are not changed.

| hd_struct->in_flight
---------------------------
sda1 | 0
sda2 | 1
---------------------------
3. The request is finished and blk_account_io_done() is called. In this case,
sda2's hd_struct->in_flight, not a sda1's one, is decremented.

| hd_struct->in_flight
---------------------------
sda1 | -1
sda2 | 1
---------------------------
The patch fixes the problem.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
---
block/blk-core.c | 13 ++++++++-----
block/blk-merge.c | 2 +-
include/linux/blkdev.h | 1 +
3 files changed, 10 insertions(+), 6 deletions(-)
Index: linux-2.6.36-rc7/block/blk-core.c
===================================================================
--- linux-2.6.36-rc7.orig/block/blk-core.c 2010-10-15 09:21:37.000000000 +0900
+++ linux-2.6.36-rc7/block/blk-core.c 2010-10-15 09:44:23.000000000 +0900
@@ -64,13 +64,15 @@ static void drive_stat_acct(struct reque
return;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));

- if (!new_io)
+ if (!new_io) {
+ part = rq->part;
part_stat_inc(cpu, part, merges[rw]);
- else {
+ } else {
+ part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
part_round_stats(cpu, part);
part_inc_in_flight(part, rw);
+ rq->part = part;
}

part_stat_unlock();
@@ -128,6 +130,7 @@ void blk_rq_init(struct request_queue *q
rq->ref_count = 1;
rq->start_time = jiffies;
set_start_time_ns(rq);
+ rq->part = NULL;
}
EXPORT_SYMBOL(blk_rq_init);

@@ -1759,7 +1762,7 @@ static void blk_account_io_completion(st
int cpu;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
+ part = req->part;
part_stat_add(cpu, part, sectors[rw], bytes >> 9);
part_stat_unlock();
}
@@ -1779,7 +1782,7 @@ static void blk_account_io_done(struct r
int cpu;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
+ part = req->part;

part_stat_inc(cpu, part, ios[rw]);
part_stat_add(cpu, part, ticks[rw], duration);
Index: linux-2.6.36-rc7/block/blk-merge.c
===================================================================
--- linux-2.6.36-rc7.orig/block/blk-merge.c 2010-10-07 05:39:52.000000000 +0900
+++ linux-2.6.36-rc7/block/blk-merge.c 2010-10-15 09:38:45.000000000 +0900
@@ -343,7 +343,7 @@ static void blk_account_io_merge(struct
int cpu;

cpu = part_stat_lock();
- part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
+ part = rq->part;

part_round_stats(cpu, part);
part_dec_in_flight(part, rq_data_dir(req));
Index: linux-2.6.36-rc7/include/linux/blkdev.h
===================================================================
--- linux-2.6.36-rc7.orig/include/linux/blkdev.h 2010-10-15 09:21:37.000000000 +0900
+++ linux-2.6.36-rc7/include/linux/blkdev.h 2010-10-15 09:26:22.000000000 +0900
@@ -115,6 +115,7 @@ struct request {
void *elevator_private3;

struct gendisk *rq_disk;
+ struct hd_struct *part;
unsigned long start_time;
#ifdef CONFIG_BLK_CGROUP
unsigned long long start_time_ns;



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