Re: [PATCH v4 9/9] hugetlb_cgroup: Add hugetlb_cgroup reservation docs

[shuah <shuah@kernel.org>]

On 9/10/19 5:31 PM, Mina Almasry wrote:
> Add docs for how to use hugetlb_cgroup reservations, and their behavior.
> 
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> Acked-by: Hillf Danton <hdanton@sina.com>
> ---
>   .../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
> --- a/Documentation/admin-guide/cgroup-v1/hugetlb.rst
> +++ b/Documentation/admin-guide/cgroup-v1/hugetlb.rst
> @@ -2,13 +2,6 @@
>   HugeTLB Controller
>   ==================
> 
> -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
>   files include::
> @@ -40,11 +37,76 @@ files include::
>     hugetlb.1GB.max_usage_in_bytes
>     hugetlb.1GB.usage_in_bytes
>     hugetlb.1GB.failcnt
> +  hugetlb.1GB.reservation_limit_in_bytes
> +  hugetlb.1GB.reservation_max_usage_in_bytes
> +  hugetlb.1GB.reservation_usage_in_bytes
> +  hugetlb.1GB.reservation_failcnt
>     hugetlb.64KB.limit_in_bytes
>     hugetlb.64KB.max_usage_in_bytes
>     hugetlb.64KB.usage_in_bytes
>     hugetlb.64KB.failcnt
> +  hugetlb.64KB.reservation_limit_in_bytes
> +  hugetlb.64KB.reservation_max_usage_in_bytes
> +  hugetlb.64KB.reservation_usage_in_bytes
> +  hugetlb.64KB.reservation_failcnt
>     hugetlb.32MB.limit_in_bytes
>     hugetlb.32MB.max_usage_in_bytes
>     hugetlb.32MB.usage_in_bytes
>     hugetlb.32MB.failcnt
> +  hugetlb.32MB.reservation_limit_in_bytes
> +  hugetlb.32MB.reservation_max_usage_in_bytes
> +  hugetlb.32MB.reservation_usage_in_bytes
> +  hugetlb.32MB.reservation_failcnt
> +
> +
> +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
> +charging.
> +
> +Shared hugetlb memory is only uncharged when it is unreseved or deallocated.

Spelling?

> +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.
> +

Is this enforced? What happens when attempt is made to set fault limit
on a cgroup that has reservation limit and vice versa.

> +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.

What would be the scenarios where setting both could be useful? Please 
explain.
> --
> 2.23.0.162.g0b9fbb3734-goog
> 

thanks,
-- Shuah