[RESEND,PATCH v4 0/3] Try to release mmap_lock temporarily in smaps_rollup
From: Chinwen Chang
Date: Sun Oct 04 2020 - 22:40:32 EST
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.
Change since v1:
- If current VMA is freed after dropping the lock, it will return
- incomplete result. To fix this issue, refine the code flow as
- suggested by Steve. [1]
Change since v2:
- When getting back the mmap lock, the address where you stopped last
- time could now be in the middle of a vma. Add one more check to handle
- this case as suggested by Michel. [2]
Change since v3:
- last_stopped is easily confused with last_vma_end. Replace it with
- a direct call to smap_gather_stats(vma, &mss, last_vma_end) as
- suggested by Steve. [3]
[1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@xxxxxxx/
[2] https://lore.kernel.org/lkml/CANN689FtCsC71cjAjs0GPspOhgo_HRj+diWsoU1wr98YPktgWg@xxxxxxxxxxxxxx/
[3] https://lore.kernel.org/lkml/db0d40e2-72f3-09d5-c162-9c49218f128f@xxxxxxx/
Chinwen Chang (3):
mmap locking API: add mmap_lock_is_contended()
mm: smaps*: extend smap_gather_stats to support specified beginning
mm: proc: smaps_rollup: do not stall write attempts on mmap_lock
fs/proc/task_mmu.c | 96 +++++++++++++++++++++++++++++++++++----
include/linux/mmap_lock.h | 5 ++
2 files changed, 92 insertions(+), 9 deletions(-)