Re: [PATCH v2 0/2] Try to release mmap_lock temporarily in smaps_rollup

[Michel Lespinasse]

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.