On Fri, 28 Jan 2022, Miklos Szeredi wrote: > On Thu, 27 Jan 2022 at 03:47, NeilBrown <neilb@xxxxxxx> wrote: > > > > inode_congested() reports if the backing-device for the inode is > > congested. Few bdi report congestion any more, only ceph, fuse, and > > nfs. Having support just for those is unlikely to be useful. > > > > The places which test inode_congested() or it variants like > > inode_write_congested(), avoid initiating IO if congestion is present. > > We now have to rely on other places in the stack to back off, or abort > > requests - we already do for everything except these 3 filesystems. > > > > So remove inode_congested() and related functions, and remove the call > > sites, assuming that inode_congested() always returns 'false'. > > Looks to me this is going to "break" fuse; e.g. readahead path will go > ahead and try to submit more requests, even if the queue is getting > congested. In this case the readahead submission will eventually > block, which is counterproductive. > > I think we should *first* make sure all call sites are substituted > with appropriate mechanisms in the affected filesystems and as a last > step remove the superfluous bdi congestion mechanism. > > You are saying that all fs except these three already have such > mechanisms in place, right? Can you elaborate on that? Not much. I haven't looked into how other filesystems cope, I just know that they must because no other filesystem ever has a congested bdi (with one or two minor exceptions, like filesystems over drbd). Surely read-ahead should never block. If it hits congestion, the read-ahead request should simply fail. block-based filesystems seem to set REQ_RAHEAD which might get mapped to REQ_FAILFAST_MASK, though I don't know how that is ultimately used. Maybe fuse and others should continue to track 'congestion' and reject read-ahead requests when congested. Maybe also skip WB_SYNC_NONE writes.. Or maybe this doesn't really matter in practice... I wonder if we can measure the usefulness of congestion. Thanks, NeilBrown
