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

From: Theodore Ts'o
Date: Wed Jun 17 2015 - 17:49:17 EST

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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