Re: FIO: kjournald blocked for more than 120 seconds

From: Lin Ming
Date: Thu Jun 26 2008 - 05:35:37 EST

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Jens,

How about below patch?

If cfqd->sync_flight > 0, we still allow async queue to be dispached,
and its slice time would be decreased. So sync queue never starve async
queue.

Introduce a sysfs interface cfq_slice_async_penalty to adjust the
percentages of slice that will be decreased.

Signed-off-by: Lin Ming <ming.m.lin@xxxxxxxxx>
---
block/cfq-iosched.c | 33 ++++++++++++++++++++++++++++++++-
1 files changed, 32 insertions(+), 1 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index d01b411..8bdc931 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -26,6 +26,12 @@ static const int cfq_slice_sync = HZ / 10;
static int cfq_slice_async = HZ / 25;
static const int cfq_slice_async_rq = 2;
static int cfq_slice_idle = HZ / 125;
+/*
+ * penalty of dispatching async queue
+ * if there are unfinished requests in sync queue
+ * by default, 50% of time slice decreased
+ */
+static int cfq_slice_async_penalty = 50;

/*
* offset from end of service tree
@@ -114,6 +120,7 @@ struct cfq_data {
unsigned int cfq_slice[2];
unsigned int cfq_slice_async_rq;
unsigned int cfq_slice_idle;
+ unsigned int cfq_slice_async_penalty;

struct list_head cic_list;
};
@@ -288,6 +295,25 @@ static inline int cfq_slice_used(struct cfq_queue
*cfqq)
}

/*
+ * Get the slice time to be decreased
+ * when async queue is dispatched and
+ * there are unfinished requests in sync queue
+ */
+static inline int
+cfq_async_penalty(struct cfq_data *cfqd, struct cfq_queue *cfqq)
+{
+ int penalty;
+
+ if (cfq_cfqq_sync(cfqq))
+ return 0;
+
+ penalty = (cfqq->slice_end - jiffies) *
+ cfqd->cfq_slice_async_penalty / 100;
+
+ return penalty;
+}
+
+/*
* Lifted from AS - choose which of rq1 and rq2 that is best served
now.
* We choose the request that is closest to the head right now.
Distance
* behind the head is penalized and only allowed to a certain extent.
@@ -1103,7 +1129,7 @@ static int cfq_dispatch_requests(struct
request_queue *q, int force)
}

if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
- break;
+ cfqq->slice_end -= cfq_async_penalty(cfqd, cfqq);

cfq_clear_cfqq_must_dispatch(cfqq);
cfq_clear_cfqq_wait_request(cfqq);
@@ -2143,6 +2169,7 @@ static void *cfq_init_queue(struct request_queue
*q)
cfqd->cfq_slice[1] = cfq_slice_sync;
cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
cfqd->cfq_slice_idle = cfq_slice_idle;
+ cfqd->cfq_slice_async_penalty = cfq_slice_async_penalty;

return cfqd;
}
@@ -2211,6 +2238,7 @@ SHOW_FUNCTION(cfq_slice_idle_show,
cfqd->cfq_slice_idle, 1);
SHOW_FUNCTION(cfq_slice_sync_show, cfqd->cfq_slice[1], 1);
SHOW_FUNCTION(cfq_slice_async_show, cfqd->cfq_slice[0], 1);
SHOW_FUNCTION(cfq_slice_async_rq_show, cfqd->cfq_slice_async_rq, 0);
+SHOW_FUNCTION(cfq_slice_async_penalty_show,
cfqd->cfq_slice_async_penalty, 0);
#undef SHOW_FUNCTION

#define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV) \
@@ -2242,6 +2270,8 @@ STORE_FUNCTION(cfq_slice_sync_store,
&cfqd->cfq_slice[1], 1, UINT_MAX, 1);
STORE_FUNCTION(cfq_slice_async_store, &cfqd->cfq_slice[0], 1, UINT_MAX,
1);
STORE_FUNCTION(cfq_slice_async_rq_store, &cfqd->cfq_slice_async_rq, 1,
UINT_MAX, 0);
+STORE_FUNCTION(cfq_slice_async_penalty_store,
&cfqd->cfq_slice_async_penalty, 0,
+ 100, 0);
#undef STORE_FUNCTION

#define CFQ_ATTR(name) \
@@ -2257,6 +2287,7 @@ static struct elv_fs_entry cfq_attrs[] = {
CFQ_ATTR(slice_async),
CFQ_ATTR(slice_async_rq),
CFQ_ATTR(slice_idle),
+ CFQ_ATTR(slice_async_penalty),
__ATTR_NULL
};

Lin Ming

On Tue, 2008-06-17 at 11:31 +0200, Jens Axboe wrote:
> On Tue, Jun 17 2008, Lin Ming wrote:
> >
> > On Tue, 2008-06-17 at 10:36 +0200, Jens Axboe wrote:
> > > On Tue, Jun 17 2008, Zhang, Yanmin wrote:
> > > > >>-----Original Message-----
> > > > >>From: Jens Axboe [mailto:jens.axboe@xxxxxxxxxx]
> > > > >>Sent: Tuesday, June 17, 2008 3:30 AM
> > > > >>To: Lin, Ming M
> > > > >>Cc: Zhang, Yanmin; Linux Kernel Mailing List
> > > > >>Subject: Re: FIO: kjournald blocked for more than 120 seconds
> > > > >>
> > > > >>On Mon, Jun 16 2008, Lin Ming wrote:
> > > > >>> Hi, Jens
> > > > >>>
> > > > >>> When runnig FIO benchmark, kjournald blocked for more than 120
> > > > seconds.
> > > > >>> Detailed root cause analysis and proposed solutions as below.
> > > > >>>
> > > > >>> Any comment is appreciated.
> > > > >>>
> > > > >>> Hardware Environment
> > > > >>> ---------------------
> > > > >>> 13 SEAGATE ST373307FC disks in a JBOD, connected by a Qlogic ISP2312
> > > > >>> Fibe Channel HBA.
> > > > >>>
> > > > >>> Bug description
> > > > >>> ----------------
> > > > >>> fio vsync random read 4K in 13 disks, 4 processes per disk, fio
> > > > global
> > > > >>> paramter as below,
> > > > >>> Tested 4 IO schedulers, issue is only seen in CFQ.
> > > > >>>
> > > > >>> INFO: task kjournald:20558 blocked for more than 120 seconds.
> > > > >>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > > > >>> message.
> > > > >>> kjournald D ffff810010820978 6712 20558 2
> > > > >>> ffff81022ddb1d10 0000000000000046 ffff81022e7baa10 ffffffff803ba6f2
> > > > >>> ffff81022ecd0000 ffff8101e6dc9160 ffff81022ecd0348 000000008048b6cb
> > > > >>> 0000000000000086 ffff81022c4e8d30 0000000000000000 ffffffff80247537
> > > > >>> Call Trace:
> > > > >>> [<ffffffff803ba6f2>] kobject_get+0x12/0x17
> > > > >>> The disks of my testing machine are tagged devices, so the CFQ idle
> > > > >>> window is disabled. In other words, the active queue of tagged
> > > > >>> devices(cfqd->hw_tag=1) never idle for a new request.
> > > > >>>
> > > > >>> This causes active queue be expired immediately if it's empty,
> > > > although
> > > > >>> it has not run out of time. CFQ will select next queue as active
> > > > queue.
> > > > >>> In this testcase, there are thousands of FIO read requests in sync
> > > > >>> queues, only a few write requests by journal_write_commit_record in
> > > > >>> async queues.
> > > > >>>
> > > > >>> In the other hand, all processes use the default io class and
> > > > priority.
> > > > >>> They share the async queue for the same device, but have their own
> > > > sync
> > > > >>> queue, so the sync queue number is 4 while asyn queue number is just
> > > > 1
> > > > >>> for the same device.
> > > > >>>
> > > > >>> So sync queue has much more chances be selected as new active queue
> > > > than
> > > > >>> async queue.
> > > > >>>
> > > > >>> Sync queues do not idle and they are dispatched all the time. This
> > > > leads
> > > > >>> to many unfinished requests in external queue,
> > > > >>> namely, cfqd->sync_flight > 0.
> > > > >>>
> > > > >>> static int cfq_dispatch_requests (...) {
> > > > >>> ....
> > > > >>> while ((cfqq = cfq_select_queue(cfqd)) != NULL) {
> > > > >>> ....
> > > > >>> if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
> > > > >>> break;
> > > > >>> ....
> > > > >>> __cfq_dispatch_requests(cfqq)
> > > > >>> }
> > > > >>> ....
> > > > >>> }
> > > > >>>
> > > > >>> When cfq_select_queue selects the async queue which includes
> > > > kjournald's
> > > > >>> write request, this selected async queue will never be dispatched
> > > > since
> > > > >>> cfqd->sync_flight > 0, so kjournald is blocked.
> > > > >>>
> > > > >>> Proposed 3 solutions
> > > > >>> ------------------
> > > > >>> 1. Do not check cfqd->sync_flight
> > > > >>>
> > > > >>> - if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
> > > > >>> - break;
> > > > >>>
> > > > >>> 2. If we do need to check cfqd->sync_flight, then for tagged
> > > > devices, we
> > > > >>> should give a little more chances to async queue to be dispatched.
> > > > >>>
> > > > >>> @@ -1102,7 +1102,7 @@ static int cfq_dispatch_requests(struct
> > > > >>> request_queue *q, int force)
> > > > >>> break;
> > > > >>> }
> > > > >>>
> > > > >>> - if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq))
> > > > >>> + if (cfqd->sync_flight && !cfq_cfqq_sync(cfqq) && !
> > > > >>> cfqd->hw_tag)
> > > > >>> break;
> > > > >>>
> > > > >>> 3. Force write request issued by journal_write_commit_record as sync
> > > > >>> request. As a matter of fact, it looks like most write requests
> > > > >>> submitted by kjournald is async request. We need convert them to
> > > > sync
> > > > >>> requests.
> > > > >>
> > > > >>Thanks for the very detailed analysis of the problem, complete with
> > > > >>suggestions. While I think that any code that does:
> > > > >>
> > > > >> submit async io
> > > > >> wait for it
> > > > >>
> > > > >>should be issuing sync IO (or, better, automatically upgrade the
> > > > request
> > > > >>from async -> sync), we cannot rely on that.
> > > > [YM] We can talk case by case. We could convert some important async io
> > > > codes
> > > > to sync io codes at least. For example, kjournald calls
> > > > sync_dirty_buffer what
> > > > we captured in this case.
> > >
> > > I agree, we should fix the obvious cases. My point was merely that there
> > > will probably always be missed cases, so we should attempt to handle it
> > > in the scheduler as well. Does the below buffer patch make it any
> > > better?
> >
> > Yes, kjournald blocked issue is gone with below patch applied.
>
> I think it's obviously the right thing to do, but I'm also a bit worried
> about applying it so close to 2.6.26 release. OTOH, we need to do
> SOMETHING for 2.6.26 release, so...
>
> >
> > Lin Ming
> >
> > >
> > > > Another case is writeback. If processes do mmapped I/O and they might
> > > > stop in
> > > > page fault to wait writeback finishing. Or a buffer write might trigger
> > > > a dirty
> > > > page balance. As the latest kernel is more aggressive to start
> > > > writeback, it might
> > > > be an issue now.
> > >
> > > Sync process getting stuck in async writeout is another problem of the
> > > same variety.
> > >
> > > diff --git a/fs/buffer.c b/fs/buffer.c
> > > index a073f3f..1957a8f 100644
> > > --- a/fs/buffer.c
> > > +++ b/fs/buffer.c
> > > @@ -2978,7 +2978,7 @@ int sync_dirty_buffer(struct buffer_head *bh)
> > > if (test_clear_buffer_dirty(bh)) {
> > > get_bh(bh);
> > > bh->b_end_io = end_buffer_write_sync;
> > > - ret = submit_bh(WRITE, bh);
> > > + ret = submit_bh(WRITE_SYNC, bh);
> > > wait_on_buffer(bh);
> > > if (buffer_eopnotsupp(bh)) {
> > > clear_buffer_eopnotsupp(bh);
> > >
> >
>

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Next message: Bartlomiej Zolnierkiewicz: "Re: [PATCH 1/4] ide-pmac: media-bay support fixes"
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Next in thread: Jens Axboe: "Re: FIO: kjournald blocked for more than 120 seconds"
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