On Thu, Aug 08, 2019 at 03:03:00PM -0400, Johannes Weiner wrote: > psi tracks the time tasks wait for refaulting pages to become > uptodate, but it does not track the time spent submitting the IO. The > submission part can be significant if backing storage is contended or > when cgroup throttling (io.latency) is in effect - a lot of time is Or the wbt is throttling. > spent in submit_bio(). In that case, we underreport memory pressure. > > Annotate submit_bio() to account submission time as memory stall when > the bio is reading userspace workingset pages. PAtch looks fine to me, but it raises another question w.r.t. IO stalls and reclaim pressure feedback to the vm: how do we make use of the pressure stall infrastructure to track inode cache pressure and stalls? With the congestion_wait() and wait_iff_congested() being entire non-functional for block devices since 5.0, there is no IO load based feedback going into memory reclaim from shrinkers that might require IO to free objects before they can be reclaimed. This is directly analogous to page reclaim writing back dirty pages from the LRU, and as I understand it one of things the PSI is supposed to be tracking. Lots of workloads create inode cache pressure and often it can dominate the time spent in memory reclaim, so it would seem to me that having PSI only track/calculate pressure and stalls from LRU pages misses a fair chunk of the memory pressure and reclaim stalls that can be occurring. Any thoughts of how we might be able to integrate more of the system caches into the PSI infrastructure, Johannes? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
