On Fri, Aug 02, 2019 at 02:11:53PM +0000, Chris Mason wrote: > On 1 Aug 2019, at 19:58, Dave Chinner wrote: > I can't really see bio->b_ioprio working without the rest of the IO > controller logic creating a sensible system, That's exactly the problem we need to solve. The current situation is ... untenable. Regardless of whether the io.latency controller works well, the fact is that the wbt subsystem is active on -all- configurations and the way it "prioritises" is completely broken. > framework to define weights etc. My question is if it's worth trying > inside of the wbt code, or if we should just let the metadata go > through. As I said, that doesn't solve the problem. We /want/ critical journal IO to have higher priority that background metadata writeback. Just ignoring REQ_META doesn't help us there - it just moves the priority inversion to blocking on request queue tags. > Tejun reminded me that in a lot of ways, swap is user IO and it's > actually fine to have it prioritized at the same level as user IO. We I think that's wrong. Swap *in* could have user priority but swap *out* is global as there is no guarantee that the page being swapped belongs to the user context that is reclaiming memory. Lots of other user and kernel reclaim contexts may be waiting on that swap to complete, so it's important that swap out is not arbitrarily delayed or susceptible to priority inversions. i.e. swap out must take priority over swap-in and other user IO because that IO may require allocation to make progress via swapping to free "user" file data cached in memory.... > don't want to let a low prio app thrash the drive swapping things in and > out all the time, Low priority apps will be throttled on *swap in* IO - i.e. by their incoming memory demand. High priority apps should be swapping out low priority app memory if there are shortages - that's what priority defines.... > other higher priority processes aren't waiting for the memory. This > depends on the cgroup config, so wrt your current patches it probably > sounds crazy, but we have a lot of data around this from the fleet. I'm not using cgroups. Core infrastructure needs to work without cgroups being configured to confine everything in userspace to "safe" bounds, and right now just running things in the root cgroup doesn't appear to work very well at all. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
