On 19/9/24 23:10, Jan Kara wrote: > Hi Joseph! > > On Wed 18-09-19 14:35:15, Joseph Qi wrote: >> On 19/9/17 18:32, Ritesh Harjani wrote: >>> Hello, >>> >>> This patch series is based on the upstream discussion with Jan >>> & Joseph @ [1]. >>> It is based on top of Matthew's v3 ext4 iomap patch series [2] >>> >>> Patch-1: Adds the ext4_ilock/unlock APIs and also replaces all >>> inode_lock/unlock instances from fs/ext4/* >>> >>> For now I already accounted for trylock/lock issue symantics >>> (which was discussed here [3]) in the same patch, >>> since the this whole patch was around inode_lock/unlock API, >>> so I thought it will be best to address that issue in the same patch. >>> However, kindly let me know if otherwise. >>> >>> Patch-2: Commit msg of this patch describes in detail about >>> what it is doing. >>> In brief - we try to first take the shared lock (instead of exclusive >>> lock), unless it is a unaligned_io or extend_io. Then in >>> ext4_dio_write_checks(), if we start with shared lock, we see >>> if we can really continue with shared lock or not. If not, then >>> we release the shared lock then acquire exclusive lock >>> and restart ext4_dio_write_checks(). >>> >>> >>> Tested against few xfstests (with dioread_nolock mount option), >>> those ran fine (ext4 & generic). >>> >>> I tried testing performance numbers on my VM (since I could not get >>> hold of any real h/w based test device). I could test the fact >>> that earlier we were trying to do downgrade_write() lock, but with >>> this patch, that path is now avoided for fio test case >>> (as reported by Joseph in [4]). >>> But for the actual results, I am not sure if VM machine testing could >>> really give the reliable perf numbers which we want to take a look at. >>> Though I do observe some form of perf improvements, but I could not >>> get any reliable numbers (not even with the same list of with/without >>> patches with which Joseph posted his numbers [1]). >>> >>> >>> @Joseph, >>> Would it be possible for you to give your test case a run with this >>> patches? That will be really helpful. >>> >>> Branch for this is hosted at below tree. >>> >>> https://github.com/riteshharjani/linux/tree/ext4-ilock-RFC >>> >> I've tested your branch, the result is: >> mounting with dioread_nolock, it behaves the same like reverting >> parallel dio reads + dioread_nolock; >> while mounting without dioread_nolock, no improvement, or even worse. >> Please refer the test data below. >> >> fio -name=parallel_dio_reads_test -filename=/mnt/nvme0n1/testfile >> -direct=1 -iodepth=1 -thread -rw=randrw -ioengine=psync -bs=$bs >> -size=20G -numjobs=8 -runtime=600 -group_reporting >> >> w/ = with parallel dio reads >> w/o = reverting parallel dio reads > > This is with 16c54688592ce8 "ext4: Allow parallel DIO reads" reverted, > right? Yes, actually, it also reverts the related patches: Revert "ext4: remove EXT4_STATE_DIOREAD_LOCK flag" Revert "ext4: fix off-by-one error when writing back pages before dio read" Revert "ext4: Allow parallel DIO reads" > >> w/o+ = reverting parallel dio reads + dioread_nolock >> ilock = ext4-ilock-RFC >> ilock+ = ext4-ilock-RFC + dioread_nolock >> >> bs=4k: >> -------------------------------------------------------------- >> | READ | WRITE | >> -------------------------------------------------------------- >> w/ | 30898KB/s,7724,555.00us | 30875KB/s,7718,479.70us | >> -------------------------------------------------------------- >> w/o | 117915KB/s,29478,248.18us | 117854KB/s,29463,21.91us | >> -------------------------------------------------------------- > > I'm really surprised by the numbers here. They would mean that when DIO > read takes i_rwsem exclusive lock instead of shared, it is a win for your > workload... Argh, now checking code in fs/direct-io.c I think I can see the > difference. The trick in do_blockdev_direct_IO() is: > > if (iov_iter_rw(iter) == READ && (dio->flags & DIO_LOCKING)) > inode_unlock(dio->inode); > if (dio->is_async && retval == 0 && dio->result && > (iov_iter_rw(iter) == READ || dio->result == count)) > retval = -EIOCBQUEUED; > else > dio_await_completion(dio); > > So actually only direct IO read submission is protected by i_rwsem with > DIO_LOCKING. Actual waiting for sync DIO read happens with i_rwsem dropped. > > After some thought I think the best solution for this is to just finally > finish the conversion of ext4 so that dioread_nolock is the only DIO path. > With i_rwsem held in shared mode even for "unlocked" DIO, it should be > actually relatively simple and most of the dances with unwritten extents > shouldn't be needed anymore. > > Honza >
