linux-kernel - Re: [PATCH 2/4] block, bfq: avoid moving bfqq to it's parent bfqg

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Message-ID: <20211221115247.GE24748@quack2.suse.cz>
Date:   Tue, 21 Dec 2021 12:52:47 +0100
From:   Jan Kara <jack@...e.cz>
To:     "yukuai (C)" <yukuai3@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, tj@...nel.org, axboe@...nel.dk,
        paolo.valente@...aro.org, fchecconi@...il.com,
        avanzini.arianna@...il.com, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH 2/4] block, bfq: avoid moving bfqq to it's parent bfqg

On Tue 21-12-21 19:08:44, yukuai (C) wrote:
> 在 2021/12/21 18:16, Jan Kara 写道:
> > On Tue 21-12-21 11:21:33, Yu Kuai wrote:
> > > Moving bfqq to it's parent bfqg is pointless.
> > > 
> > > Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> > 
> > Did you notice that this is happening often enough that the check is worth
> > it? Where do we do this?
> > 
> 
> I noticed that this will happend when root group is offlined:
> 
> bfq_pd_offline
>  bfq_put_async_queues
>   __bfq_put_async_bfqq
>    bfq_bfqq_move
> 
> I'm not sure if there are other situations. I think bfq_bfqq_move()
> is not happening often itself, thus the checking won't affect
> performance.

Yeah, OK, I was just wondering. I guess there's no harm in doing this
check. Maybe add a comment that this can sometimes happen when dealing with
the root cgroup. And then feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR