linux-kernel - Re: [RFC PATCH 0/3] block: Fix fsync slowness with CFQ cgroups

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Date:	Tue, 28 Jun 2011 18:42:57 +0400
From:	Konstantin Khlebnikov <khlebnikov@...allels.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jaxboe@...ionio.com" <jaxboe@...ionio.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>
Subject: Re: [RFC PATCH 0/3] block: Fix fsync slowness with CFQ cgroups

Vivek Goyal wrote:
> On Tue, Jun 28, 2011 at 03:00:39PM +0400, Konstantin Khlebnikov wrote:
>> Vivek Goyal wrote:
>>> This patch series seems to be working for me. I did testing for ext4 only.
>>> This series is based on for-3.1/core branch of Jen's block tree.
>>> Konstantin, can you please give it a try and see if it fixes your
>>> issue.
>>
>> It works for me too, for ext3 and ext4, on top 3.0-rc5, after these trivial fixes:
>>
>> --- a/block/cfq-iosched.c
>> +++ b/block/cfq-iosched.c
>> @@ -1511,7 +1519,7 @@ static void cfq_add_rq_rb(struct request *rq)
>>           * if that happens, put the alias on the dispatch list
>>           */
>>          while ((__alias = elv_rb_add(&cfqq->sort_list, rq)) != NULL)
>> -               cfq_dispatch_insert(cfqd->queue, __alias);
>> +               cfq_dispatch_insert(cfqd->queue, __alias, false);
>>
>>          if (!cfq_cfqq_on_rr(cfqq))
>>                  cfq_add_cfqq_rr(cfqd, cfqq);
>> @@ -3797,12 +3797,11 @@ cfq_set_depends_on_task(struct request_queue *q, struct task_struct *tsk)
>>           */
>>          rcu_read_lock();
>>          if (task_blkio_cgroup(current) == task_blkio_cgroup(tsk))
>> -               return;
>> -       rcu_read_unlock();
>> +               goto out_unlock_rcu;
>>
>>          cic = cfq_cic_lookup(cfqd, current->io_context);
>>          if (!cic)
>> -               return;
>> +               goto out_unlock_rcu;
>
> You have done this change because you want to keep cfq_cic_lookup() also
> in rcu read side critical section? I am assuming that it works even
> without this. Though keeping it under rcu is probably more correct as
> cic objects are freed in rcu manner.

No, your just forgot to relese rcu in case task_blkio_cgroup(current) == task_blkio_cgroup(tsk)

>
> Thanks
> Vivek

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