On Wed, Aug 12, 2020 at 7:14 PM Chinwen Chang <chinwen.chang@xxxxxxxxxxxx> wrote: > Recently, we have observed some janky issues caused by unpleasantly long > contention on mmap_lock which is held by smaps_rollup when probing large > processes. To address the problem, we let smaps_rollup detect if anyone > wants to acquire mmap_lock for write attempts. If yes, just release the > lock temporarily to ease the contention. > > smaps_rollup is a procfs interface which allows users to summarize the > process's memory usage without the overhead of seq_* calls. Android uses it > to sample the memory usage of various processes to balance its memory pool > sizes. If no one wants to take the lock for write requests, smaps_rollup > with this patch will behave like the original one. > > Although there are on-going mmap_lock optimizations like range-based locks, > the lock applied to smaps_rollup would be the coarse one, which is hard to > avoid the occurrence of aforementioned issues. So the detection and > temporary release for write attempts on mmap_lock in smaps_rollup is still > necessary. I do not mind extending the mmap lock API as needed. However, in the past I have tried adding rwsem_is_contended to mlock(), and later to mm_populate() paths, and IIRC gotten pushback on it both times. I don't feel strongly on this, but would prefer if someone else approved the rwsem_is_contended() use case. Couple related questions, how many VMAs are we looking at here ? Would need_resched() be workable too ? -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.
