[Devel] Re: [PATCH 03/24] io-controller: bfq support of in-class preemption

[Jerome Marchand]

Vivek Goyal wrote:
> On Tue, Jul 28, 2009 at 01:44:32PM +0200, Jerome Marchand wrote:
>> Vivek Goyal wrote:
>>> Hi Jerome,
>>>
>>> Thanks for testing it out. I could also reproduce the issue.
>>>
>>> I had assumed that RT queue will always preempt non-RT queue and hence if
>>> there is an RT ioq/request pending, the sd->next_entity will point to
>>> itself and any queue which is preempting it has to be on same service
>>> tree.
>>>
>>> But in your test case it looks like that RT async queue is pending and 
>>> there is some sync BE class IO going on. It looks like that CFQ allows
>>> sync queue preempting async queue irrespective of class, so in this case
>>> sync BE class reader will preempt async RT queue and that's where my
>>> assumption is broken and we see BUG_ON() hitting.
>>>
>>> Can you please tryout following patch. It is a quick patch and requires
>>> more testing. It solves the crash but still does not solve the issue of
>>> sync queue always preempting async queues irrespective of class. In
>>> current scheduler we always schedule the RT queue first (whether it be
>>> sync or async). This problem requires little more thought.
>> I've tried it: I can't reproduce the issue anymore and I haven't seen any
>> other problem so far.
>> By the way, what is the expected result regarding fairness among different
>> groups when IO from different classes are run on each group? For instance,
>> if we have RT IO going on on one group, BE IO on an other and Idle IO on a
>> third group, what is the expected result: should the IO time been shared
>> fairly between the groups or should RT IO have priority? As it is now, the
>> time is shared fairly between BE and RT groups and the last group running
>> Idle IO hardly get any time.
>>
> 
> Hi Jerome,
> 
> If there are two groups RT and BE, I would expect RT group to get all the
> bandwidth as long as it is backlogged and starve the BE group.

I wasn't clear enough. I meant the class of the process as set by ionice, not
the class of the cgroup. That is, of course, only an issue when using CFQ.

> 
> I ran quick test of two dd readers. One reader is in RT group and other is
> in BE group. I do see that RT group runs away with almost all the BW.
> 
> group1 time=8:16 2479 group1 sectors=8:16 457848
> group2 time=8:16 103  group2 sectors=8:16 18936
> 
> Note that when group1 (RT) finished it had got 2479 ms of disk time while
> group2 (BE) got only 103 ms.
> 
> Can you send details of your test. It should not be fair sharing between
> RT and BE group.

Setup:

$ mount -t cgroup -o io,blkio none /cgroup
$ mkdir /cgroup/test1 /cgroup/test2 /cgroup/test3
$ echo 1000 > /cgroup/test1/io.weight
$ echo 1000 > /cgroup/test2/io.weight
$ echo 1000 > /cgroup/test3/io.weight

Test:
$ echo 3 > /proc/sys/vm/drop_caches

$ ionice -c 1 dd if=/tmp/io-controller-test3 of=/dev/null &
$ echo $! > /cgroup/test1/tasks

$ ionice -c 2 dd if=/tmp/io-controller-test1 of=/dev/null &
$ echo $! > /cgroup/test2/tasks

$ ionice -c 3 dd if=/tmp/io-controller-test2 of=/dev/null &
$ echo $! > /cgroup/test3/tasks


> 
> Thanks
> Vivek

Jerome
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