linux-kernel - Re: [PATCH BUGFIX V2 1/2] block, bfq: put async queues for root bfq groups too

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Message-Id: <D75A2C53-9012-481B-986B-88F9040A3FB3@linaro.org>
Date:   Wed, 10 Jan 2018 07:13:45 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Guoqing Jiang <gqjiang@...e.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>, linus.walleij@...aro.org,
        bfq-iosched@...glegroups.com, oleksandr@...alenko.name,
        162996@...denti.unimore.it,
        Davide Ferrari <davideferrari8@...il.com>
Subject: Re: [PATCH BUGFIX V2 1/2] block, bfq: put async queues for root bfq
 groups too



> Il giorno 10 gen 2018, alle ore 02:41, Guoqing Jiang <gqjiang@...e.com> ha scritto:
> 
> 
> 
> On 01/09/2018 05:27 PM, Paolo Valente wrote:
>> For each pair [device for which bfq is selected as I/O scheduler,
>> group in blkio/io], bfq maintains a corresponding bfq group. Each such
>> bfq group contains a set of async queues, with each async queue
>> created on demand, i.e., when some I/O request arrives for it.  On
>> creation, an async queue gets an extra reference, to make sure that
>> the queue is not freed as long as its bfq group exists.  Accordingly,
>> to allow the queue to be freed after the group exited, this extra
>> reference must released on group exit.
>> 
>> The above holds also for a bfq root group, i.e., for the bfq group
>> corresponding to the root blkio/io root for a given device. Yet, by
>> mistake, the references to the existing async queues of a root group
>> are not released when the latter exits. This causes a memory leak when
>> the instance of bfq for a given device exits. In a similar vein,
>> bfqg_stats_xfer_dead is not executed for a root group.
>> 
>> This commit fixes bfq_pd_offline so that the latter executes the above
>> missing operations for a root group too.
>> 
>> Reported-by: Holger Hoffstätte <holger@...lied-asynchrony.com>
>> Reported-by: Guoqing Jiang <gqjiang@...e.com>
>> Signed-off-by: Davide Ferrari <davideferrari8@...il.com>
>> Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
>> ---
>>  block/bfq-cgroup.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>> 
>> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
>> index da1525ec4c87..d819dc77fe65 100644
>> --- a/block/bfq-cgroup.c
>> +++ b/block/bfq-cgroup.c
>> @@ -775,10 +775,11 @@ static void bfq_pd_offline(struct blkg_policy_data *pd)
>>  	unsigned long flags;
>>  	int i;
>>  +	spin_lock_irqsave(&bfqd->lock, flags);
>> +
>>  	if (!entity) /* root group */
>> -		return;
>> +		goto put_async_queues;
>>  -	spin_lock_irqsave(&bfqd->lock, flags);
>>  	/*
>>  	 * Empty all service_trees belonging to this group before
>>  	 * deactivating the group itself.
>> @@ -809,6 +810,8 @@ static void bfq_pd_offline(struct blkg_policy_data *pd)
>>  	}
>>    	__bfq_deactivate_entity(entity, false);
>> +
>> +put_async_queues:
>>  	bfq_put_async_queues(bfqd, bfqg);
>>    	spin_unlock_irqrestore(&bfqd->lock, flags);
> 
> With this change, bfqg_stats_xfer_dead will be called even entity is not existed,

Actually, the entity associated with the bfq group being offlined
exists even if the local variable entity is NULL here.  Simply, that
variable gets NULL if the bfq group is the bfq root group for a
device.

> is it necessary?

No, I opted for this solution to not shake the code too much, and
considering that
- bfqg_stats_xfer_dead simply does nothing if applied
to a root group
- who knows, in the future that function may need
do be invoked for a root group too.

Yet, if you guys think that it would be cleaner to not invoke
bfqg_stats_xfer_dead at all for a root group, I'll change the code
accordingly (this would introduce a little asymmetry with
cfq_pd_offline, which invokes cfqg_stats_xfer_dead unconditionally).

Thanks,
Paolo

> Thanks.
> 
> Regards,
> Guoqing