On Tue 07-05-24 09:16:39, Kemeng Shi wrote: > > Hi Jan, > on 5/3/2024 5:30 PM, Jan Kara wrote: > > On Thu 25-04-24 21:17:22, Kemeng Shi wrote: > >> The wb_calc_thresh is supposed to calculate wb's share of bg_thresh in > >> global domain. To calculate wb's share of bg_thresh in cgroup domain, > >> it's more reasonable to use __wb_calc_thresh in which way we calculate > >> dirty_thresh in cgroup domain in balance_dirty_pages(). > >> > >> Consider following domain hierarchy: > >> global domain (> 20G) > >> / \ > >> cgroup domain1(10G) cgroup domain2(10G) > >> | | > >> bdi wb1 wb2 > >> Assume wb1 and wb2 has the same bandwidth. > >> We have global domain bg_thresh > 2G, cgroup domain bg_thresh 1G. > >> Then we have: > >> wb's thresh in global domain = 2G * (wb bandwidth) / (system bandwidth) > >> = 2G * 1/2 = 1G > >> wb's thresh in cgroup domain = 1G * (wb bandwidth) / (system bandwidth) > >> = 1G * 1/2 = 0.5G > >> At last, wb1 and wb2 will be limited at 0.5G, the system will be limited > >> at 1G which is less than global domain bg_thresh 2G. > > > > This was a bit hard to understand for me so I'd rephrase it as: > > > > wb_calc_thresh() is calculating wb's share of bg_thresh in the global > > domain. However in case of cgroup writeback this is not the right thing to > > do. Consider the following domain hierarchy: > > > > global domain (> 20G) > > / \ > > cgroup1 (10G) cgroup2 (10G) > > | | > > bdi wb1 wb2 > > > > and assume wb1 and wb2 have the same bandwidth and the background threshold > > is set at 10%. The bg_thresh of cgroup1 and cgroup2 is going to be 1G. Now > > because wb_calc_thresh(mdtc->wb, mdtc->bg_thresh) calculates per-wb > > threshold in the global domain as (wb bandwidth) / (domain bandwidth) it > > returns bg_thresh for wb1 as 0.5G although it has nobody to compete against > > in cgroup1. > > > > Fix the problem by calculating wb's share of bg_thresh in the cgroup > > domain. > Thanks for improving the changelog. As this was merged into -mm and > mm-unstable tree, I'm not sure if a new patch is needed. If there is > anything I should do, please let me konw. Thanks. No need to do anything here. Andrew has picked up these updates. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
