On 11/29/22 00:25, Christoph Hellwig wrote: > On Sat, Nov 26, 2022 at 11:55:50AM +0900, Damien Le Moal wrote: >> Taking a blktrace of a simple fio run on a block device file using >> libaio and iodepth > 1 reveals that asynchronous writes are executed as >> sync writes, that is, REQ_SYNC is set for the write BIOs. >> >> Fix this by modifying dio_bio_write_op() to set REQ_SYNC only for IOs >> that are indeed synchronous ones and set REQ_IDLE only for asynchronous >> IOs. > > Well, REQ_SYNC is used for I/O that some is actively waiting for, > which includs aio/io_uring I/O unlike buffered writback. OK. But then the semantic of REQ_SYNC should really be more clearly defined. Always setting it for bdev direct write IOs regardless of the iocb type is telling me that we should not need it at all, especially considering that for bdev direct IO reads, it is the reverse: REQ_SYNC is never set. > So I don't think this should be changed. OK. But then what ? Looking again at the block layer use of REQ_SYNC, I do not see anything super relevant. wbt_should_throttle() use it to detect direct writes. Some other places set that flag but it does not seem to actually be used/tested anywhere. What am I missing ? -- Damien Le Moal Western Digital Research
