[PATCH 0/5] hugetlbfs: Disable PMD sharing for large systems

From: Waiman Long
Date: Wed Sep 11 2019 - 11:06:08 EST

A customer with large SMP systems (up to 16 sockets) with application
that uses large amount of static hugepages (~500-1500GB) are experiencing
random multisecond delays. These delays was caused by the long time it
took to scan the VMA interval tree with mmap_sem held.

To fix this problem while perserving existing behavior as much as
possible, we need to allow timeout in down_write() and disabling PMD
sharing when it is taking too long to do so. Since a transaction can
involving touching multiple huge pages, timing out for each of the huge
page interactions does not completely solve the problem. So a threshold
is set to completely disable PMD sharing if too many timeouts happen.

The first 4 patches of this 5-patch series adds a new
down_write_timedlock() API which accepts a timeout argument and return
true is locking is successful or false otherwise. It works more or less
than a down_write_trylock() but the calling thread may sleep.

The last patch implements the timeout mechanism as described above. With
the patched kernel installed, the customer confirmed that the problem
was gone.

Waiman Long (5):
locking/rwsem: Add down_write_timedlock()
locking/rwsem: Enable timeout check when spinning on owner
locking/osq: Allow early break from OSQ
locking/rwsem: Enable timeout check when staying in the OSQ
hugetlbfs: Limit wait time when trying to share huge PMD

include/linux/fs.h | 7 ++
include/linux/osq_lock.h | 13 +--
include/linux/rwsem.h | 4 +-
kernel/locking/lock_events_list.h | 1 +
kernel/locking/mutex.c | 2 +-
kernel/locking/osq_lock.c | 12 +-
kernel/locking/rwsem.c | 183 +++++++++++++++++++++++++-----
mm/hugetlb.c | 24 +++-
8 files changed, 201 insertions(+), 45 deletions(-)