This patch set adds support of iopoll for dm devices. Several months ago, I also sent a patch set adding support of iopoll for dm devices [1]. The old patch set implement this by polling all polling mode hardware queues of all underlying target devices unconditionally, no matter which hardware queue the bio is enqueued into. Ming Lei pointed out that this implementation may have performance issue. Mike Snitzer also discussed and helpt a lot on the design issue. At that time, we agreed that this feature should be implemented on the basis of split-bio tracking, since the bio to the dm device could be split into multiple split bios to the underlying target devices. This patch set actually implement the split-bio tracking part. Regrettably this implementation is quite coarse and original. Quite code refactoring is introduced to the block core, since device mapper also calls methods provided by block core to split bios. The new fields are directly added into 'struct bio' structure. So this is just an RFC version and there may be quite a lot design issues be fronted on. I just implement a version quickly and would like to share with you my thoughts and problems at hand. This implementation works but has poor performance. Awkwardly the performance of direct 8k randread decreases ~20% on a 8k-striped dm-stripe device. The first 5 patches prepare the introduction of iopoll for dm device. Patch 6 is the core part and implements a mechanism of tracking split bios for bio-based device. Patch 7 just enables this feature. [1] https://patchwork.kernel.org/project/linux-block/cover/20201020065420.124885-1-jefflexu@xxxxxxxxxxxxxxxxx/ [Design Notes] - recursive way or non-recursive way? The core of the split-bio tracking mechanism is that, a list is maintained in the top bio (the original bio submitted to the dm device), which is actually used to maintain all valid cookies of all split bios. This is actually a non-recursive way to implement the tracking mechanism. On the contrary, the recursive way is that, maintain the split bios at each dm layer. DM device can be build into a device stack. For example, considering the following device stack, ``` dm0(bio 0) dm1(bio 1) dm2(bio 2) nvme0(bio 3) nvme1(bio 4) ``` The non-recursive way is that bio 3/4 are directly maintained as a list beneath bio 0. The recursive way is that, bio 3 is maintained as a list beneath bio 1 and bio 4 is maintained as a list beneath bio 2, while bio 1/2 are maintained as a list beneath bio 0. The reason why I choose the non-recursive way is that, we need call blk_bio_poll() or something recursively if it's implemented in the recursive way, and I worry this would consume up the stack space if the device stack is too deep. After all bio_list is used to prevent the dm device using up stack space when submitting bio. - why embed new fields into struct bio directly? Mike Snitzer had ever suggested that the newly added fields could be integrated into the clone bio allocated by dm subsystem through bio_set. There're 3 reasons why I directly embed these fields into struct bio: 1. The implementation difference of DM and MD DM subsystem indeed allocate clone bio itself, in which case we could allocate extra per-bio space for specific usage. However MD subsystem doesn't allocate extra clone bio, and just uses the bio structure originally submitted to MD device as the bio structure submitted to the underlying target device. (At least raid0 works in this way.) 2. In the previously mentioned non-recursive way of iopoll, a list containing all valid cookies should be maintained. For convenience, I just put this list in the top bio (the original bio structure submitted to the dm device). This original bio structure is allocated by the upper layer, e.g. filesystem, and is out of control of DM subsystem. (Of course we could resolve this problem technically, e.g., put these newlly added fields into the corresponding dm_io structure of the top bio.) 3. As a quick implementation, I just put these fields into struct bio directly. Obviously more design issues need to be considered when it comes into the formal version. Jeffle Xu (7): block: move definition of blk_qc_t to types.h block: add helper function fetching gendisk from queue block: add iopoll method for non-mq device block: define blk_qc_t as uintptr_t dm: always return BLK_QC_T_NONE for bio-based device block: track cookies of split bios for bio-based device dm: add support for IO polling block/bio.c | 8 ++ block/blk-core.c | 163 ++++++++++++++++++++++++++++++++++- block/blk-mq.c | 70 ++------------- drivers/md/dm-table.c | 28 ++++++ drivers/md/dm.c | 27 +++--- include/linux/blk-mq.h | 3 + include/linux/blk_types.h | 41 ++++++++- include/linux/blkdev.h | 3 + include/linux/fs.h | 2 +- include/linux/types.h | 3 + include/trace/events/kyber.h | 6 +- 11 files changed, 270 insertions(+), 84 deletions(-) -- 2.27.0 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
