[097/181] mmap locking API: add mmap_lock_is_contended()

Commit Message

Andrew Morton Oct. 13, 2020, 11:53 p.m. UTC

From: Chinwen Chang <[e-mail]>
Subject: mmap locking API: add mmap_lock_is_contended()

Patch series "Try to release mmap_lock temporarily in smaps_rollup", v4.

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.


This patch (of 3):

Add new API to query if someone wants to acquire mmap_lock for write
attempts.

Using this instead of rwsem_is_contended makes it more tolerant of future
changes to the lock type.

Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Chinwen Chang <[e-mail]>
Reviewed-by: Steven Price <[e-mail]>
Acked-by: Michel Lespinasse <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Daniel Jordan <[e-mail]>
Cc: Daniel Kiss <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: Huang Ying <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Jimmy Assarsson <[e-mail]>
Cc: Laurent Dufour <[e-mail]>
Cc: "Matthew Wilcox (Oracle)" <[e-mail]>
Cc: Matthias Brugger <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 include/linux/mmap_lock.h |    5 +++++
 1 file changed, 5 insertions(+)

Patch

--- a/include/linux/mmap_lock.h~mmap-locking-api-add-mmap_lock_is_contended
+++ a/include/linux/mmap_lock.h
@@ -87,4 +87,9 @@  static inline void mmap_assert_write_loc
 	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
 }
 
+static inline int mmap_lock_is_contended(struct mm_struct *mm)
+{
+	return rwsem_is_contended(&mm->mmap_lock);
+}
+
 #endif /* _LINUX_MMAP_LOCK_H */