On 2019/2/7 11:10 上午, Dave Chinner wrote: > On Thu, Feb 07, 2019 at 10:38:58AM +0800, Coly Li wrote: >> On 2019/2/7 10:26 上午, Dave Chinner wrote: >>> On Thu, Feb 07, 2019 at 01:24:25AM +0100, Andre Noll wrote: >>>> On Thu, Feb 07, 10:43, Dave Chinner wrote >>>>> File data readahead: REQ_RAHEAD >>>>> Metadata readahead: REQ_META | REQ_RAHEAD >>>>> >>>>> drivers/md/bcache/request.c::check_should_bypass(): >>>>> >>>>> /* >>>>> * Flag for bypass if the IO is for read-ahead or background, >>>>> * unless the read-ahead request is for metadata (eg, for gfs2). >>>>> */ >>>>> if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) && >>>>> !(bio->bi_opf & REQ_PRIO)) >>>>> goto skip; >>>>> >>>>> bcache needs fixing - it thinks REQ_PRIO means metadata IO. That's >>>>> wrong - REQ_META means it's metadata IO, and so this is a bcache >>>>> bug. >>>> >>>> Do you think 752f66a75abad is bad (ha!) and should be reverted? >>> >>> Yes, that change is just broken. From include/linux/blk_types.h: >>> >>> __REQ_META, /* metadata io request */ >>> __REQ_PRIO, /* boost priority in cfq */ >>> >>> >> >> Hi Dave, >> >>> i.e. REQ_META means that it is a metadata request, REQ_PRIO means it >>> is a "high priority" request. Two completely different things, often >>> combined, but not interchangeable. >> >> I found in file system metadata IO, most of time REQ_META and REQ_PRIO >> are tagged together for bio, but XFS seems not use REQ_PRIO. > > Yes, that's correct, because we don't specifically prioritise > metadata IO over data IO. > >> Is there any basic principle for when should these tags to be used or >> not ? > > Yes. > >> e.g. If REQ_META is enough for meta data I/O, why REQ_PRIO is used >> too. > > REQ_META is used for metadata. REQ_PRIO is used to communicate to > the lower layers that the submitter considers this IO to be more > important that non REQ_PRIO IO and so dispatch should be expedited. > > IOWs, if the filesystem considers metadata IO to be more important > that user data IO, then it will use REQ_PRIO | REQ_META rather than > just REQ_META. > > Historically speaking, REQ_PRIO was a hack for CFQ to get it to > dispatch journal IO from a different thread without waiting for a > time slice to expire. In the XFs world, we just said "don't use CFQ, > it's fundametnally broken for highly concurrent applications" and > didn't bother trying to hack around the limitations of CFQ. > > These days REQ_PRIO is only used by the block layer writeback > throttle, but unlike bcache it considers both REQ_META and REQ_PRIO > to mean the same thing. > > REQ_META, OTOH, is used by BFQ and blk-cgroup to detect metadata > IO and don't look at REQ_PRIO at all. So, really, REQ_META is for > metadata, not REQ_PRIO. REQ_PRIO looks like it should just go away. > >> And if REQ_PRIO is necessary, why it is not used in fs/xfs/ code ? > > It's not necessary, it's just an /optimisation/ that some > filesystems make and some IO schedulers used to pay attention to. It > looks completely redundant now. Hi Dave, Thanks for your detailed explanation. This is great hint from view of file system developer :-) I just compose a fix, hope it makes better. -- Coly Li
