[PATCH v4 9/9] hugetlb_cgroup: Add hugetlb_cgroup reservation docs
From: Mina Almasry
Date: Tue Sep 10 2019 - 19:32:20 EST
Add docs for how to use hugetlb_cgroup reservations, and their behavior.
Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
Acked-by: Hillf Danton <hdanton@xxxxxxxx>
.../admin-guide/cgroup-v1/hugetlb.rst | 84 ++++++++++++++++---
1 file changed, 73 insertions(+), 11 deletions(-)
diff --git a/Documentation/admin-guide/cgroup-v1/hugetlb.rst b/Documentation/admin-guide/cgroup-v1/hugetlb.rst
index a3902aa253a96..cc6eb859fc722 100644
@@ -2,13 +2,6 @@
-The HugeTLB controller allows to limit the HugeTLB usage per control group and
-enforces the controller limit during page fault. Since HugeTLB doesn't
-support page reclaim, enforcing the limit at page fault time implies that,
-the application will get SIGBUS signal if it tries to access HugeTLB pages
-beyond its limit. This requires the application to know beforehand how much
-HugeTLB pages it would require for its use.
HugeTLB controller can be created by first mounting the cgroup filesystem.
# mount -t cgroup -o hugetlb none /sys/fs/cgroup
@@ -28,10 +21,14 @@ process (bash) into it.
Brief summary of control files::
- hugetlb.<hugepagesize>.limit_in_bytes # set/show limit of "hugepagesize" hugetlb usage
- hugetlb.<hugepagesize>.max_usage_in_bytes # show max "hugepagesize" hugetlb usage recorded
- hugetlb.<hugepagesize>.usage_in_bytes # show current usage for "hugepagesize" hugetlb
- hugetlb.<hugepagesize>.failcnt # show the number of allocation failure due to HugeTLB limit
+ hugetlb.<hugepagesize>.reservation_limit_in_bytes # set/show limit of "hugepagesize" hugetlb reservations
+ hugetlb.<hugepagesize>.reservation_max_usage_in_bytes # show max "hugepagesize" hugetlb reservations recorded
+ hugetlb.<hugepagesize>.reservation_usage_in_bytes # show current reservations for "hugepagesize" hugetlb
+ hugetlb.<hugepagesize>.reservation_failcnt # show the number of allocation failure due to HugeTLB reservation limit
+ hugetlb.<hugepagesize>.limit_in_bytes # set/show limit of "hugepagesize" hugetlb faults
+ hugetlb.<hugepagesize>.max_usage_in_bytes # show max "hugepagesize" hugetlb usage recorded
+ hugetlb.<hugepagesize>.usage_in_bytes # show current usage for "hugepagesize" hugetlb
+ hugetlb.<hugepagesize>.failcnt # show the number of allocation failure due to HugeTLB usage limit
For a system supporting three hugepage sizes (64k, 32M and 1G), the control
@@ -40,11 +37,76 @@ files include::
+1. Reservation limits
+The HugeTLB controller allows to limit the HugeTLB reservations per control
+group and enforces the controller limit at reservation time. Reservation limits
+are superior to Page fault limits (see section 2), since Reservation limits are
+enforced at reservation time, and never causes the application to get SIGBUS
+signal. Instead, if the application is violating its limits, then it gets an
+error on reservation time, i.e. the mmap or shmget return an error.
+2. Page fault limits
+The HugeTLB controller allows to limit the HugeTLB usage (page fault) per
+control group and enforces the controller limit during page fault. Since HugeTLB
+doesn't support page reclaim, enforcing the limit at page fault time implies
+that, the application will get SIGBUS signal if it tries to access HugeTLB
+pages beyond its limit. This requires the application to know beforehand how
+much HugeTLB pages it would require for its use.
+3. Caveats with shared memory
+a. Charging and uncharging:
+For shared hugetlb memory, both hugetlb reservation and usage (page faults) are
+charged to the first task that causes the memory to be reserved or faulted,
+and all subsequent uses of this reserved or faulted memory is done without
+Shared hugetlb memory is only uncharged when it is unreseved or deallocated.
+This is usually when the hugetlbfs file is deleted, and not when the task that
+caused the reservation or fault has exited.
+b. Interaction between reservation limit and fault limit.
+Generally, it's not recommended to set both of the reservation limit and fault
+limit in a cgroup. For private memory, the fault usage cannot exceed the
+reservation usage, so if you set both, one of those limits will be useless.
+For shared memory, a cgroup's fault usage may be greater than its reservation
+usage, so some care needs to be taken. Consider this example:
+- Task A reserves 4 pages in a shared hugetlbfs file. Cgroup A will get
+ 4 reservations charged to it and no faults charged to it.
+- Task B reserves and faults the same 4 pages as Task A. Cgroup B will get no
+ reservation charge, but will get charged 4 faulted pages. If Cgroup B's limit
+ is less than 4, then Task B will get a SIGBUS.
+For the above scenario, it's not recommended for the userspace to set both
+reservation limits and fault limits, but it is still allowed to in case it sees
+some use for it.