On Thu, 02 Apr 2020 10:53:20 +1100 NeilBrown wrote: > > PF_LESS_THROTTLE exists for loop-back nfsd, and a similar need in the > loop block driver, where a daemon needs to write to one bdi in > order to free up writes queued to another 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. > > 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. > > This patch changes the heuristic to ignore the global limits and > consider only the limit relevant to the bdi being written to. This > approach is already available for BDI_CAP_STRICTLIMIT users (fuse) and > should not introduce surprises. This has the desired result of > protecting the task from the consequences of large amounts of dirty data > queued for other devices. > > This approach of "only consider the target bdi" 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, > - remove the 25% bonus that that flag gives, and > - imposes 'strictlimit' handling for any process with PF_LOCAL_THROTTLE > set. /* * The strictlimit feature is a tool preventing mistrusted filesystems * from growing a large number of dirty pages before throttling. For Based on the comment snippet, I suspect it is applicable to IO flushers unless they are likely generating tons of dirty pages. If they are, however, cutting their bonuses seem questionable. > > 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. > > Signed-off-by: NeilBrown <neilb@xxxxxxx> > =2D-- Hrm corrupted delivery? > drivers/block/loop.c | 2 +- > fs/nfsd/vfs.c | 9 +++++---- > include/linux/sched.h | 2 +- > kernel/sys.c | 2 +- > mm/page-writeback.c | 10 ++++++---- > mm/vmscan.c | 4 ++-- > 6 files changed, 16 insertions(+), 13 deletions(-)
