linux-kernel - Re: [PATCH 04/10] block: initial patch for on-stack per-task plugging

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Date:	Thu, 17 Mar 2011 10:44:51 +0100
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Shaohua Li <shaohua.li@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH 04/10] block: initial patch for on-stack per-task  plugging

On 2011-03-17 04:19, Shaohua Li wrote:
> On Thu, 2011-03-17 at 09:00 +0800, Shaohua Li wrote:
>> On Thu, 2011-03-17 at 01:31 +0800, Vivek Goyal wrote:
>>> On Wed, Mar 16, 2011 at 04:18:30PM +0800, Shaohua Li wrote:
>>>> 2011/1/22 Jens Axboe <jaxboe@...ionio.com>:
>>>>> Signed-off-by: Jens Axboe <jaxboe@...ionio.com>
>>>>> ---
>>>>>  block/blk-core.c          |  357 ++++++++++++++++++++++++++++++++------------
>>>>>  block/elevator.c          |    6 +-
>>>>>  include/linux/blk_types.h |    2 +
>>>>>  include/linux/blkdev.h    |   30 ++++
>>>>>  include/linux/elevator.h  |    1 +
>>>>>  include/linux/sched.h     |    6 +
>>>>>  kernel/exit.c             |    1 +
>>>>>  kernel/fork.c             |    3 +
>>>>>  kernel/sched.c            |   11 ++-
>>>>>  9 files changed, 317 insertions(+), 100 deletions(-)
>>>>>
>>>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>>>> index 960f12c..42dbfcc 100644
>>>>> --- a/block/blk-core.c
>>>>> +++ b/block/blk-core.c
>>>>> @@ -27,6 +27,7 @@
>>>>>  #include <linux/writeback.h>
>>>>>  #include <linux/task_io_accounting_ops.h>
>>>>>  #include <linux/fault-inject.h>
>>>>> +#include <linux/list_sort.h>
>>>>>
>>>>>  #define CREATE_TRACE_POINTS
>>>>>  #include <trace/events/block.h>
>>>>> @@ -213,7 +214,7 @@ static void blk_delay_work(struct work_struct *work)
>>>>>
>>>>>        q = container_of(work, struct request_queue, delay_work.work);
>>>>>        spin_lock_irq(q->queue_lock);
>>>>> -       q->request_fn(q);
>>>>> +       __blk_run_queue(q);
>>>>>        spin_unlock_irq(q->queue_lock);
>>>>>  }
>>>> Hi Jens,
>>>> I have some questions about the per-task plugging. Since the request
>>>> list is per-task, and each task delivers its requests at finish flush
>>>> or schedule. But when one cpu delivers requests to global queue, other
>>>> cpus don't know. This seems to have problem. For example:
>>>> 1. get_request_wait() can only flush current task's request list,
>>>> other cpus/tasks might still have a lot of requests, which aren't sent
>>>> to request_queue.
>>>
>>> But very soon these requests will be sent to request queue as soon task
>>> is either scheduled out or task explicitly flushes the plug? So we might
>>> wait a bit longer but that might not matter in general, i guess. 
>> Yes, I understand there is just a bit delay. I don't know how severe it
>> is, but this still could be a problem, especially for fast storage or
>> random I/O. My current tests show slight regression (3% or so) with
>> Jens's for 2.6.39/core branch. I'm still checking if it's caused by the
>> per-task plug, but the per-task plug is highly suspected.
>>
>>>> your ioc-rq-alloc branch is for this, right? Will it
>>>> be pushed to 2.6.39 too? I'm wondering if we should limit per-task
>>>> queue length. If there are enough requests there, we force a flush
>>>> plug.
>>>
>>> That's the idea jens had. But then came the question of maintaining
>>> data structures per task per disk. That makes it complicated.
>>>
>>> Even if we move the accounting out of request queue and do it say at
>>> bdi, ideally we shall to do per task per bdi accounting.
>>>
>>> Jens seemed to be suggesting that generally fluser threads are the
>>> main cluprit for submitting large amount of IO. They are already per
>>> bdi. So probably just maintain a per task limit for flusher threads.
>> Yep, flusher is the main spot in my mind. We need call more flush plug
>> for flusher thread. 
>>
>>> I am not sure what happens to direct reclaim path, AIO deep queue 
>>> paths etc.
>> direct reclaim path could build deep write queue too. It
>> uses .writepage, currently there is no flush plug there. Maybe we need
>> add flush plug in shrink_inactive_list too.
>>
>>>> 2. some APIs like blk_delay_work, which call __blk_run_queue() might
>>>> not work. because other CPUs might not dispatch their requests to
>>>> request queue. So __blk_run_queue will eventually find no requests,
>>>> which might stall devices.
>>>> Since one cpu doesn't know other cpus' request list, I'm wondering if
>>>> there are other similar issues.
>>>
>>> So again in this case if queue is empty at the time of __blk_run_queue(),
>>> then we will probably just experinece little more delay then intended
>>> till some task flushes. But should not stall the system?
>> not stall the system, but device stalls a little time.
> Jens,
> I need below patch to recover a ffsb fsync workload, which has about 30%
> regression with stack plug. 
> I guess the reason is WRITE_SYNC_PLUG doesn't work now, so if a context
> hasn't blk_plug, we lose previous plug (request merge). This suggests
> all places we use WRITE_SYNC_PLUG before (for example, kjournald) should
> have a blk_plug context.

Good point, those should be auto-converted. I'll take this patch and
double check the others. Thanks!

Does it remove that performance regression completely?

-- 
Jens Axboe

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