On Thu, Apr 26, 2018 at 12:01:18PM -0700, Tejun Heo wrote: > Hello, > > On Fri, Apr 20, 2018 at 06:06:01PM +0800, Benlong Zhang wrote: > > One problem with cgwb is how fs should treat metadata bios. > > For example in xfs, the log might be partially stuck in one > > group, leaving threads in other groups waiting for too long. > > Please refer to the linux-xfs discussion: > > "[PATCH V2] xfs: implement cgroup writeback support" > > > > One approach is to correctly tag bio->bi_css from within the > > filesystems (for xfs log should be blkcg_root), and the other > > is to skip them, but relies on REQ_META being set. > > > > It works with xfs on a cgwb porting implementation to 3.10.0. > > But really not sure about other filesystems... > > Yeah, Josef (cc'd) is working on a similar approach but in a more > generic way. > I thought Josef's work had more to do with tracking/managing dirty metadata (i.e., driving writeback/reclaim and whatnot). Perhaps I just haven't followed it close enough... how does that relate to cgroup bio ownership? The issue addressed by this patch (wrt to XFS) is that log I/O that happens to be submitted via a throttled cgroup ends up blocking unthrottled tasks because the log is essentially a global/shared mechanism. We therefore want to somehow exempt metadata I/O from throttling in that particular case. Note that I'm not familiar enough with the block layer code to know whether this patch is the right way to do that, but I think (as noted in the commit log above) that not doing the default bio association if the fs hasn't (or associating with some root group) for metadata I/O would also be sufficient. Hm? Brian > Thanks. > > -- > tejun
