linux-kernel - Re: CFQ: async queue blocks the whole system

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Date:	Sat, 11 Jun 2011 15:24:10 +0800
From:	Tao Ma <tm@....ma>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: CFQ: async queue blocks the whole system

On 06/10/2011 11:44 PM, Vivek Goyal wrote:
> On Fri, Jun 10, 2011 at 06:00:37PM +0800, Tao Ma wrote:
>> On 06/10/2011 05:14 PM, Vivek Goyal wrote:
>>> On Fri, Jun 10, 2011 at 01:48:37PM +0800, Tao Ma wrote:
>>>
>>> [..]
>>>>>> btw, reverting the patch doesn't work. I can still get the livelock.
>>>
>>> What test exactly you are running. I am primarily interested in whether
>>> you still get the hung task timeout warning where a writer is waiting on
>>> get_request_wait() for more than 120 secods or not.
>>>
>>> Livelock might be a different problem and for which Christoph provided
>>> a patch for XFS.
>>>
>>>>>
>>>>> Can you give following patch a try and see if it helps. On my system this
>>>>> does allow CFQ to dispatch some writes once in a while.
>>>> Sorry, this patch doesn't work in my test.
>>>
>>> Can you give me backtrace of say 15 seconds each with and without patch.
>>> I think now we must be dispatching some writes, that's a different thing
>>> that writer still sleeps more than 120 seconds because there are  way
>>> too many readers.
>>>
>>> May be we need to look into show workload tree scheduling takes place and
>>> tweak that logic a bit.
>> OK, our test cases can be downloaded for free. ;)
>> svn co http://code.taobao.org/svn/dirbench/trunk/meta_test/press/set_vs_get
>> Modify run.sh to be fit for your need. Normally within 10 mins, you will
>> get the livelock. We have a SAS disk with 15000 RPMs.
>>
>> btw, you have to mount the volume on /test since the test program are
>> not that clever. :)
> 
> Thanks for the test program. System keeps on working, that's a different
> thing that writes might not make lot of progress. 
> 
> What do you mean by livelock in your case. How do you define that?
I mean there are many tasks hang like a deadlock, but actually they can
get out finally.
> 
> Couple of times I did see hung_task warning with your test. And I also
> saw that we have most of the time starved WRITES but one in a while we
> will dispatch some writes.
yeah, that is what we get.
> 
> Having said that I will still admit that current logic can completely
> starve async writes if there are sufficient number of readers. I can
> reproduce this simply by launching lots of readers and bunch of writers
> using fio.
yeah, as I have said before, we found some async writes are delayed for
more than 500 seconds.
> 
> So I have written another patch, where I don't allow preemption of 
> async queue if it waiting for sync requests to drain and has not
> dispatched any request after having been scheduled.
> 
> This atleast makes sure that writes are not starved. But that does not
> mean that whole bunch of async writes are dispatched. In presence of
> lots of read activity, expect 1 write to be dispatched every few seconds.
> 
> Please give this patch a try and if it still does not work, please upload
> some bltraces while test is running.
OK, I will try it and return back with the test result. Great thanks for
your test and patch.

Tao
> 
> You can also run iostat on disk and should be able to see that with
> the patch you are dispatching writes more often than before.
> 
> Thanks
> Vivek
> 
> ---
>  block/cfq-iosched.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/block/cfq-iosched.c
> ===================================================================
> --- linux-2.6.orig/block/cfq-iosched.c	2011-06-10 09:13:01.000000000 -0400
> +++ linux-2.6/block/cfq-iosched.c	2011-06-10 10:02:31.850831735 -0400
> @@ -3315,8 +3315,15 @@ cfq_should_preempt(struct cfq_data *cfqd
>  	 * if the new request is sync, but the currently running queue is
>  	 * not, let the sync request have priority.
>  	 */
> -	if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
> +	if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq)) {
> +		/*
> +		 * Allow atleast one dispatch otherwise this can repeat
> +		 * and writes can be starved completely
> +		 */
> +		if (!cfqq->slice_dispatch)
> +			return false;
>  		return true;
> +	}
>  
>  	if (new_cfqq->cfqg != cfqq->cfqg)
>  		return false;
> 

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