linux-kernel - Re: [PATCH 3/3] writeback, blkio: add documentation for cgroup writeback support

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Message-ID: <20150617214852.GE4076@thunk.org>
Date:	Wed, 17 Jun 2015 17:48:52 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>, axboe@...nel.dk,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	lizefan@...wei.com, cgroups@...r.kernel.org
Subject: Re: [PATCH 3/3] writeback, blkio: add documentation for cgroup
 writeback support

On Wed, Jun 17, 2015 at 02:52:37PM -0400, Tejun Heo wrote:
> 
> Hmmm... so, overriding things *before* an bio is issued shouldn't be
> too difficult and as long as this sort of operations aren't prevalent
> we might be able to get away with just charging them against root.
> Especially if it's to avoid getting blocked on the journal which we
> already consider a shared overhead which is charged to root.  If this
> becomes large enough to require exacting charges, it'll be more
> complex but still way better than trying to raise priority on a bio
> which is already issued, which is likely to be excruciatingly painful
> if possible at all.

Yeah, just charging the overhead to root seems good enough.

I could imagine charging it to whatever cgroup the jbd/jbd2 thread
belongs to, which in turn would be the cgroup of the process that
mounted the file system.  The only problem with that is that if a
low-priority process is allowed to mount a file system, and it gets
traversed by a high priority process, the high priority process will
get impacted.  So maybe it's better to just say that it always get
charged to the root cgroup.

					- Ted
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