On Wed, Aug 12, 2020 at 7:14 PM Chinwen Chang
[off-list ref] 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.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel