From a37b0715ddf3007734c4e2424c14bc7efcdd1190 Mon Sep 17 00:00:00 2001 From: NeilBrown Date: Mon, 1 Jun 2020 21:48:18 -0700 Subject: mm/writeback: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a daemon needs to write to one bdi (the final bdi) in order to free up writes queued to another bdi (the client bdi). The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty pages, so that it can still dirty pages after other processses have been throttled. The purpose of this is to avoid deadlock that happen when the PF_LESS_THROTTLE process must write for any dirty pages to be freed, but it is being thottled and cannot write. This approach was designed when all threads were blocked equally, independently on which device they were writing to, or how fast it was. Since that time the writeback algorithm has changed substantially with different threads getting different allowances based on non-trivial heuristics. This means the simple "add 25%" heuristic is no longer reliable. The important issue is not that the daemon needs a *larger* dirty page allowance, but that it needs a *private* dirty page allowance, so that dirty pages for the "client" bdi that it is helping to clear (the bdi for an NFS filesystem or loop block device etc) do not affect the throttling of the daemon writing to the "final" bdi. This patch changes the heuristic so that the task is not throttled when the bdi it is writing to has a dirty page count below below (or equal to) the free-run threshold for that bdi. This ensures it will always be able to have some pages in flight, and so will not deadlock. In a steady-state, it is expected that PF_LOCAL_THROTTLE tasks might still be throttled by global threshold, but that is acceptable as it is only the deadlock state that is interesting for this flag. This approach of "only throttle when target bdi is busy" is consistent with the other use of PF_LESS_THROTTLE in current_may_throttle(), were it causes attention to be focussed only on the target bdi. So this patch - renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE, - removes the 25% bonus that that flag gives, and - If PF_LOCAL_THROTTLE is set, don't delay at all unless the global and the local free-run thresholds are exceeded. Note that previously realtime threads were treated the same as PF_LESS_THROTTLE threads. This patch does *not* change the behvaiour for real-time threads, so it is now different from the behaviour of nfsd and loop tasks. I don't know what is wanted for realtime. [akpm@linux-foundation.org: coding style fixes] Signed-off-by: NeilBrown Signed-off-by: Andrew Morton Reviewed-by: Jan Kara Acked-by: Chuck Lever [nfsd] Cc: Christoph Hellwig Cc: Michal Hocko Cc: Trond Myklebust Link: http://lkml.kernel.org/r/87ftbf7gs3.fsf@notabene.neil.brown.name Signed-off-by: Linus Torvalds --- fs/nfsd/vfs.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) (limited to 'fs') diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c index 0aa02eb18bd3..c3fbab1753ec 100644 --- a/fs/nfsd/vfs.c +++ b/fs/nfsd/vfs.c @@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf, if (test_bit(RQ_LOCAL, &rqstp->rq_flags)) /* - * We want less throttling in balance_dirty_pages() - * and shrink_inactive_list() so that nfs to + * We want throttling in balance_dirty_pages() + * and shrink_inactive_list() to only consider + * the backingdev we are writing to, so that nfs to * localhost doesn't cause nfsd to lock up due to all * the client's dirty pages or its congested queue. */ - current->flags |= PF_LESS_THROTTLE; + current->flags |= PF_LOCAL_THROTTLE; exp = fhp->fh_export; use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp); @@ -1037,7 +1038,7 @@ out_nfserr: nfserr = nfserrno(host_err); } if (test_bit(RQ_LOCAL, &rqstp->rq_flags)) - current_restore_flags(pflags, PF_LESS_THROTTLE); + current_restore_flags(pflags, PF_LOCAL_THROTTLE); return nfserr; } -- cgit v1.2.3