On Wed, Sep 22, 2021 at 06:46:21AM +1000, Dave Chinner wrote: > On Mon, Sep 20, 2021 at 09:54:31AM +0100, Mel Gorman wrote: > > Cc list similar to "congestion_wait() and GFP_NOFAIL" as they're loosely > > related. > > > > This is a prototype series that removes all calls to congestion_wait > > in mm/ and deletes wait_iff_congested. It's not a clever > > implementation but congestion_wait has been broken for a long time > > (https://lore.kernel.org/linux-mm/45d8b7a6-8548-65f5-cccf-9f451d4ae3d4@xxxxxxxxx/). > > Even if it worked, it was never a great idea. While excessive > > dirty/writeback pages at the tail of the LRU is one possibility that > > reclaim may be slow, there is also the problem of too many pages being > > isolated and reclaim failing for other reasons (elevated references, > > too many pages isolated, excessive LRU contention etc). > > > > This series replaces the reclaim conditions with event driven ones > > > > o If there are too many dirty/writeback pages, sleep until a timeout > > or enough pages get cleaned > > o If too many pages are isolated, sleep until enough isolated pages > > are either reclaimed or put back on the LRU > > o If no progress is being made, let direct reclaim tasks sleep until > > another task makes progress > > > > This has been lightly tested only and the testing was useless as the > > relevant code was not executed. The workload configurations I had that > > used to trigger these corner cases no longer work (yey?) and I'll need > > to implement a new synthetic workload. If someone is aware of a realistic > > workload that forces reclaim activity to the point where reclaim stalls > > then kindly share the details. > > Got a git tree pointer so I can pull it into a test kernel so I can > see what impact it has on behaviour before I try to make sense of > the code? > The current version I'm testing is at git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-reclaimcongest-v2r5 Only one test has completed and I won't be able to analyse the results in detail for a few days but it's doing *something* for the workload that is hammering reclaim 5.15.0-rc1 5.15.0-rc1 vanillamm-reclaimcongest-v2r5 Duration User 10891.30 9945.59 Duration System 5673.78 2649.43 Duration Elapsed 2402.85 2407.96 System CPU usage dropped by a lot. Workload completes runs for a fixed duration so a difference in elapsed is not interesting Ops Direct pages scanned 518791317.00 219956338.00 Ops Kswapd pages scanned 128555233.00 165439373.00 Ops Kswapd pages reclaimed 87830801.00 72216420.00 Ops Direct pages reclaimed 16114049.00 10408389.00 Ops Kswapd efficiency % 68.32 43.65 Ops Kswapd velocity 53501.15 68705.20 Ops Direct efficiency % 3.11 4.73 Ops Direct velocity 215906.66 91345.5 Ops Percentage direct scans 80.14 57.07 Ops Page writes by reclaim 4225921.00 2032865.00 Large reductions in direct pages scanned. The rate kswapd scans is roughly the same (velocity) where as direct velocity is down (presumably because it's getting throttled). Pages written from reclaim context are about halved. Kswapd scan rates are increased slightly but probably because direct reclaimers throttled. Reclaim efficiency is low but that's expected given the workload is basically trying to make it as hard as possible for reclaim to make progress. Kswapd is only getting throttled on writeback and is being woken before the timeout of 100000 1 usect_delayed=84000 reason=VMSCAN_THROTTLE_WRITEBACK 2 usect_delayed=20000 reason=VMSCAN_THROTTLE_WRITEBACK 6 usect_delayed=16000 reason=VMSCAN_THROTTLE_WRITEBACK 12 usect_delayed=12000 reason=VMSCAN_THROTTLE_WRITEBACK 17 usect_delayed=8000 reason=VMSCAN_THROTTLE_WRITEBACK 129 usect_delayed=4000 reason=VMSCAN_THROTTLE_WRITEBACK 205 usect_delayed=0 reason=VMSCAN_THROTTLE_WRITEBACK The number of throttle events for direct reclaimers were 16909 reason=VMSCAN_THROTTLE_ISOLATED 77844 reason=VMSCAN_THROTTLE_NOPROGRESS 113415 reason=VMSCAN_THROTTLE_WRITEBACK For the throttle events, 33% of them were NOPROGRESS hitting the full timeout and 33% were WRITEBACK hitting the full timeout. If anything, that would suggest increasing the max timeout as presumably they woke up uselessly like Neil had suggested. -- Mel Gorman SUSE Labs
