On Tue 30-07-19 21:11:10, Minchan Kim wrote: > On Mon, Jul 29, 2019 at 10:35:15AM +0200, Michal Hocko wrote: > > On Mon 29-07-19 17:20:52, Minchan Kim wrote: > > > On Mon, Jul 29, 2019 at 09:45:23AM +0200, Michal Hocko wrote: > > > > On Mon 29-07-19 16:10:37, Minchan Kim wrote: > > > > > In our testing(carmera recording), Miguel and Wei found unmap_page_range > > > > > takes above 6ms with preemption disabled easily. When I see that, the > > > > > reason is it holds page table spinlock during entire 512 page operation > > > > > in a PMD. 6.2ms is never trivial for user experince if RT task couldn't > > > > > run in the time because it could make frame drop or glitch audio problem. > > > > > > > > Where is the time spent during the tear down? 512 pages doesn't sound > > > > like a lot to tear down. Is it the TLB flushing? > > > > > > Miguel confirmed there is no such big latency without mark_page_accessed > > > in zap_pte_range so I guess it's the contention of LRU lock as well as > > > heavy activate_page overhead which is not trivial, either. > > > > Please give us more details ideally with some numbers. > > I had a time to benchmark it via adding some trace_printk hooks between > pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing > device is 2018 premium mobile device. > > I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the > task runs on little core even though it doesn't have any IPI and LRU > lock contention. It's already too heavy. > > If I remove activate_page, 35-40% overhead of zap_pte_range is gone > so most of overhead(about 0.7ms) comes from activate_page via > mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could > accumulate up to several ms. Thanks for this information. This is something that should be a part of the changelog. I am sorry to still poke into this because I still do not have a full understanding of what is going on and while I do not object to drop the spinlock I still suspect this is papering over a deeper problem. If mark_page_accessed is really expensive then why do we even bother to do it in the tear down path in the first place? Why don't we simply set a referenced bit on the page to reflect the young pte bit? I might be missing something here of course. -- Michal Hocko SUSE Labs
