linux-kernel - Re: [PATCH 2/9] blkcg, writeback: Add wbc->no_wbc

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Message-ID: <20190624163918.GL32376@quack2.suse.cz>
Date:   Mon, 24 Jun 2019 18:39:18 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, dsterba@...e.com, clm@...com,
        josef@...icpanda.com, axboe@...nel.dk, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 2/9] blkcg, writeback: Add wbc->no_wbc_acct

On Mon 24-06-19 05:58:56, Tejun Heo wrote:
> Hello, Jan.
> 
> On Mon, Jun 24, 2019 at 10:21:30AM +0200, Jan Kara wrote:
> > OK, now I understand. Just one more question: So effectively, you are using
> > wbc->no_wbc_acct to pass information from btrfs code to btrfs code telling
> > it whether IO should or should not be accounted with wbc_account_io().
> 
> Yes.
> 
> > Wouldn't it make more sense to just pass this information internally
> > within btrfs? Granted, if this mechanism gets more widespread use by other
> > filesystems, then probably using wbc flag makes more sense. But I'm not
> > sure if this isn't a premature generalization...
> 
> The btrfs async issuers end up using generic writeback path and uses
> the generic wbc owner mechanisms so that ios are attached to the right
> cgroup too.  So, I kinda prefer to provide a generic mechanism from
> wbc side.

OK, I can live with that. We just have to be kind of careful so that people
just don't sprinkle no_wbc_acct writeback around because they don't know
better. Maybe you could at least add comment to no_wbc_acct mentioning that
this is for the cases where writeback has already been accounted for?

> That said, the names are a bit misleading and I think it'd
> be better to rename them to something more explicit, e.g. sth along
> the line of wbc_update_cgroup_owner() and wbc->no_cgroup_owner.  What
> do you think?

Yeah, renaming would probably make things clearer as well.

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