linux-kernel - Re: [PATCH-cgroup v5 1/2] cgroup/rstat: Optimize cgroup_rstat_updated

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Date:   Fri, 1 Dec 2023 13:09:57 -0500
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, Joe Mario <jmario@...hat.com>,
        Sebastian Jug <sejug@...hat.com>,
        Yosry Ahmed <yosryahmed@...gle.com>
Subject: Re: [PATCH-cgroup v5 1/2] cgroup/rstat: Optimize
 cgroup_rstat_updated_list()


On 12/1/23 12:37, Tejun Heo wrote:
> On Thu, Nov 30, 2023 at 03:43:26PM -0500, Waiman Long wrote:
>> The current design of cgroup_rstat_cpu_pop_updated() is to traverse
>> the updated tree in a way to pop out the leaf nodes first before
>> their parents. This can cause traversal of multiple nodes before a
>> leaf node can be found and popped out. IOW, a given node in the tree
>> can be visited multiple times before the whole operation is done. So
>> it is not very efficient and the code can be hard to read.
>>
>> With the introduction of cgroup_rstat_updated_list() to build a list
>> of cgroups to be flushed first before any flushing operation is being
>> done, we can optimize the way the updated tree nodes are being popped
>> by pushing the parents first to the tail end of the list before their
>> children. In this way, most updated tree nodes will be visited only
>> once with the exception of the subtree root as we still need to go
>> back to its parent and popped it out of its updated_children list.
>> This also makes the code easier to read.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
> Applied to cgroup/for-6.8 with a small comment edit.
>
> ...
>> + * Iteratively traverse down the cgroup_rstat_cpu updated tree level by
>> + * level and push all the parents first before their next level children
>> + * into a singly linked list built from the tail backward like "pushing"
>> + * cgroups into a stack. The parent is by the caller.
> I found the last sentence a bit difficult to understand and changed it to
> "The root is pushed by the caller." That's what you meant, right?

Yes, you are right. Thanks for the edit.

Cheers,
Longman