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Message-ID: <9ec6f0ad-6236-4514-43ba-f202e948b905@redhat.com>
Date: Thu, 1 Jun 2023 09:30:40 -0400
From: Waiman Long <longman@...hat.com>
To: Randy Dunlap <rdunlap@...radead.org>, Tejun Heo <tj@...nel.org>,
Zefan Li <lizefan.x@...edance.com>,
Johannes Weiner <hannes@...xchg.org>,
Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
Juri Lelli <juri.lelli@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Frederic Weisbecker <frederic@...nel.org>,
Mrunal Patel <mpatel@...hat.com>,
Ryan Phillips <rphillips@...hat.com>,
Brent Rowsell <browsell@...hat.com>,
Peter Hunt <pehunt@...hat.com>, Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v2 5/6] cgroup/cpuset: Documentation update for partition
On 5/31/23 19:11, Randy Dunlap wrote:
> Hi,
>
> Just a few small nits below:
Thanks for catching that.
Cheers,
Longman
>
> On 5/31/23 09:34, Waiman Long wrote:
>> This patch updates the cgroup-v2.rst file to include information about
>> the new "cpuset.cpus.reserve" control file as well as the new remote
>> partition.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>> Documentation/admin-guide/cgroup-v2.rst | 92 +++++++++++++++++++++----
>> 1 file changed, 79 insertions(+), 13 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
>> index f67c0829350b..3e9351c2cd27 100644
>> --- a/Documentation/admin-guide/cgroup-v2.rst
>> +++ b/Documentation/admin-guide/cgroup-v2.rst
>> @@ -2215,6 +2215,38 @@ Cpuset Interface Files
>>
>> Its value will be affected by memory nodes hotplug events.
>>
>> + cpuset.cpus.reserve
>> + A read-write multiple values file which exists only on root
>> + cgroup.
>> +
>> + It lists all the CPUs that are reserved for adjacent and remote
>> + partitions created in the system. See the next section for
>> + more information on what an adjacent or remote partitions is.
>> +
>> + Creation of adjacent partition does not require touching this
>> + control file as CPU reservation will be done automatically.
>> + In order to create a remote partition, the CPUs needed by the
>> + remote partition has to be written to this file first.
>> +
>> + Due to the fact that "cpuset.cpus.reserve" holds reserve CPUs
>> + that can be used by multiple partitions and automatic reservation
>> + may also race with manual reservation, an extension prefixes of
>> + "+" and "-" are allowed for this file to reduce race.
>> +
>> + A "+" prefix can be used to indicate a list of additional
>> + CPUs that are to be added without disturbing the CPUs that are
>> + originally there. For example, if its current value is "3-4",
>> + echoing ""+5" to it will change it to "3-5".
> "+5"
>
>> +
>> + Once a remote partition is destroyed, its CPUs have to be
>> + removed from this file or no other process can use them. A "-"
>> + prefix can be used to remove a list of CPUs from it. However,
>> + removing CPUs that are currently used in existing partitions
>> + may cause those partitions to become invalid. A single "-"
>> + character without any number can be used to indicate removal
>> + of all the free CPUs not yet allocated to any partitions to
>> + avoid accidental partition invalidation.
>> +
>> cpuset.cpus.partition
>> A read-write single value file which exists on non-root
>> cpuset-enabled cgroups. This flag is owned by the parent cgroup
>> @@ -2228,25 +2260,49 @@ Cpuset Interface Files
>> "isolated" Partition root without load balancing
>> ========== =====================================
>>
>> - The root cgroup is always a partition root and its state
>> - cannot be changed. All other non-root cgroups start out as
>> - "member".
>> + A cpuset partition is a collection of cgroups with a partition
>> + root at the top of the hierarchy and its descendants except
>> + those that are separate partition roots themselves and their
>> + descendants. A partition has exclusive access to the set of
>> + CPUs allocated to it. Other cgroups outside of that partition
>> + cannot use any CPUs in that set.
>> +
>> + There are two types of partitions - adjacent and remote. The
>> + parent of an adjacent partition must be a valid partition root.
>> + Partition roots of adjacent partitions are all clustered around
>> + the root cgroup. Creation of adjacent partition is done by
>> + writing the desired partition type into "cpuset.cpus.partition".
>> +
>> + A remote partition does not require a partition root parent.
>> + So a remote partition can be formed far from the root cgroup.
>> + However, its creation is a 2-step process. The CPUs needed
>> + by a remote partition ("cpuset.cpus" of the partition root)
>> + has to be written into "cpuset.cpus.reserve" of the root
>> + cgroup first. After that, "isolated" can be written into
>> + "cpuset.cpus.partition" of the partition root to form a remote
>> + isolated partition which is the only supported remote partition
>> + type for now.
>> +
>> + All remote partitions are terminal as adjacent partition cannot
>> + be created underneath it. With the way remote partition is
>> + formed, it is not possible to create another valid remote
>> + partition underneath it.
>> +
>> + The root cgroup is always a partition root and its state cannot
>> + be changed. All other non-root cgroups start out as "member".
>>
>> When set to "root", the current cgroup is the root of a new
>> - partition or scheduling domain that comprises itself and all
>> - its descendants except those that are separate partition roots
>> - themselves and their descendants.
>> + partition or scheduling domain.
>>
>> - When set to "isolated", the CPUs in that partition root will
>> + When set to "isolated", the CPUs in that partition will
>> be in an isolated state without any load balancing from the
>> scheduler. Tasks placed in such a partition with multiple
>> CPUs should be carefully distributed and bound to each of the
>> individual CPUs for optimal performance.
>>
>> - The value shown in "cpuset.cpus.effective" of a partition root
>> - is the CPUs that the partition root can dedicate to a potential
>> - new child partition root. The new child subtracts available
>> - CPUs from its parent "cpuset.cpus.effective".
>> + The value shown in "cpuset.cpus.effective" of a partition root is
>> + the CPUs that are dedicated to that partition and not available
>> + to cgroups outside of that partittion.
> partition.
>
>>
>> A partition root ("root" or "isolated") can be in one of the
>> two possible states - valid or invalid. An invalid partition
>> @@ -2270,8 +2326,8 @@ Cpuset Interface Files
>> In the case of an invalid partition root, a descriptive string on
>> why the partition is invalid is included within parentheses.
>>
>> - For a partition root to become valid, the following conditions
>> - must be met.
>> + For an adjacent partition root to be valid, the following
>> + conditions must be met.
>>
>> 1) The "cpuset.cpus" is exclusive with its siblings , i.e. they
>> are not shared by any of its siblings (exclusivity rule).
>> @@ -2281,6 +2337,16 @@ Cpuset Interface Files
>> 4) The "cpuset.cpus.effective" cannot be empty unless there is
>> no task associated with this partition.
>>
>> + For a remote partition root to be valid, the following conditions
>> + must be met.
>> +
>> + 1) The same exclusivity rule as adjacent partition root.
>> + 2) The "cpuset.cpus" is not empty and all the CPUs must be
>> + present in "cpuset.cpus.reserve" of the root cgroup and none
>> + of them are allocated to another partition.
>> + 3) The "cpuset.cpus" value must be present in all its ancestors
>> + to ensure proper hierarchical cpu distribution.
> CPU
>
>> +
>> External events like hotplug or changes to "cpuset.cpus" can
>> cause a valid partition root to become invalid and vice versa.
>> Note that a task cannot be moved to a cgroup with empty
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