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

[Jerome Marchand <jmarchan@redhat.com>]

Vivek Goyal wrote:
> On Tue, Jul 28, 2009 at 04:29:06PM +0200, Jerome Marchand wrote:
>> 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
>>
> 
> Ok, got it. So you have created three BE class groups and with-in those
> groups you are running job of RT, BE and IDLE type.
> 
> From group scheduling point of view, because the tree groups have got same
> class and same weight, they should get equal access to disk and with-in
> group how bandwidth is divided is left to CFQ.
> 
> Because in this case, only one task is present in each group, it should
> get all the BW available to the group. Hence, in above test case, all the
> three dd processes should get equal amount of disk time.

OK. That's how I understood it, but I wanted your confirmation.

> 
> You mentioned that RT and BE task are getting fair share but not IDLE
> task. This is a bug and probably I know where the bug is. I will debug it
> and fix it soon.

I've tested it with the last version of your patchset (v6) and the problem
was less acute (the IDLE task got about 5 times less time that RT and BE
against 50 times less with v7 patchset). I hope that helps you.

Jerome

> 
> Thanks
> Vivek