We technically support this already through io_uring, but it's implemented with a thread backend to support cases where we would block. This isn't ideal. After a few prep patches, the core of this patchset is adding support for async callbacks on page unlock. With this primitive, we can simply retry the IO operation. With io_uring, this works a lot like poll based retry for files that support it. If a page is currently locked and needed, -EIOCBQUEUED is returned with a callback armed. The callers callback is responsible for restarting the operation. With this callback primitive, we can add support for generic_file_buffered_read(), which is what most file systems end up using for buffered reads. XFS/ext4/btrfs/bdev is wired up, but probably trivial to add more. The file flags support for this by setting FMODE_BUF_RASYNC, similar to what we do for FMODE_NOWAIT. Open to suggestions here if this is the preferred method or not. In terms of results, I wrote a small test app that randomly reads 4G of data in 4K chunks from a file hosted by ext4. The app uses a queue depth of 32. If you want to test yourself, you can just use buffered=1 with ioengine=io_uring with fio. No application changes are needed to use the more optimized buffered async read. preadv for comparison: real 1m13.821s user 0m0.558s sys 0m11.125s CPU ~13% Mainline: real 0m12.054s user 0m0.111s sys 0m5.659s CPU ~32% + ~50% == ~82% This patchset: real 0m9.283s user 0m0.147s sys 0m4.619s CPU ~52% The CPU numbers are just a rough estimate. For the mainline io_uring run, this includes the app itself and all the threads doing IO on its behalf (32% for the app, ~1.6% per worker and 32 of them). Context switch rate is much smaller with the patchset, since we only have the one task performing IO. The goal here is efficiency. Async thread offload adds latency, and it also adds noticable overhead on items such as adding pages to the page cache. By allowing proper async buffered read support, we don't have X threads hammering on the same inode page cache, we have just the single app actually doing IO. Been beating on this and it's solid for me, and I'm now pretty happy with how it all turned out. Not aware of any missing bits/pieces or code cleanups that need doing. Series can also be found here: https://git.kernel.dk/cgit/linux-block/log/?h=async-buffered.3 or pull from: git://git.kernel.dk/linux-block async-buffered.3 fs/block_dev.c | 2 +- fs/btrfs/file.c | 2 +- fs/ext4/file.c | 2 +- fs/io_uring.c | 99 ++++++++++++++++++++++++++++++++++ fs/xfs/xfs_file.c | 2 +- include/linux/blk_types.h | 3 +- include/linux/fs.h | 5 ++ include/linux/pagemap.h | 64 ++++++++++++++++++++++ mm/filemap.c | 111 ++++++++++++++++++++++++-------------- 9 files changed, 245 insertions(+), 45 deletions(-) Changes since v2: - Get rid of unnecessary wait_page_async struct, just use wait_page_async - Add another prep handler, adding wake_page_match() - Use wake_page_match() in both callers Changes since v1: - Fix an issue with inline page locking - Fix a potential race with __wait_on_page_locked_async() - Fix a hang related to not setting page_match, thus missing a wakeup -- Jens Axboe
