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Re: [PATCH V2 4/4] block/loop: make loop cgroup aware

On Wed, Sep 13, 2017 at 02:01:29PM -0700, Shaohua Li wrote:
> From: Shaohua Li <shli@xxxxxx>
> loop block device handles IO in a separate thread. The actual IO
> dispatched isn't cloned from the IO loop device received, so the
> dispatched IO loses the cgroup context.
> I'm ignoring buffer IO case now, which is quite complicated.  Making the
> loop thread aware cgroup context doesn't really help. The loop device
> only writes to a single file. In current writeback cgroup
> implementation, the file can only belong to one cgroup.
> For direct IO case, we could workaround the issue in theory. For
> example, say we assign cgroup1 5M/s BW for loop device and cgroup2
> 10M/s. We can create a special cgroup for loop thread and assign at
> least 15M/s for the underlayer disk. In this way, we correctly throttle
> the two cgroups. But this is tricky to setup.
> This patch tries to address the issue. We record bio's css in loop
> command. When loop thread is handling the command, we then use the API
> provided in patch 1 to set the css for current task. The bio layer will
> use the css for new IO (from patch 3).
> Signed-off-by: Shaohua Li <shli@xxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>