From: Johannes Weiner <hannes@cmpxchg.org>
To: Kinsey Ho <kinseyho@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Yosry Ahmed <yosryahmed@google.com>,
Roman Gushchin <roman.gushchin@linux.dev>
Subject: Re: [PATCH mm-unstable v1 1/4] mm: don't hold css->refcnt during traversal
Date: Thu, 25 Jul 2024 16:43:46 -0400 [thread overview]
Message-ID: <20240725204346.GA1702603@cmpxchg.org> (raw)
In-Reply-To: <20240724190214.1108049-2-kinseyho@google.com>
On Wed, Jul 24, 2024 at 07:02:11PM +0000, Kinsey Ho wrote:
> To obtain the pointer to the saved memcg position, mem_cgroup_iter()
> currently holds css->refcnt during memcg traversal only to put
> css->refcnt at the end of the routine. This isn't necessary as an
> rcu_read_lock is already held throughout the function.
>
> Remove css->refcnt usage during traversal by leveraging RCU.
Eh, I don't know about this.
RCU ensures that the css memory isn't freed.
The tryget ensures that the css is still alive and valid.
In this case, it just so happens that the sibling linkage is also rcu
protected. But accessing random css members when the refcount is 0 is
kind of sketchy. On the other hand, the refcount is guaranteed to be
valid, and rcu + tryget is a common pattern.
What does this buy us? The tryget is cheap.
next prev parent reply other threads:[~2024-07-25 20:43 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-07-24 19:02 [PATCH mm-unstable v1 0/4] Improve mem_cgroup_iter() Kinsey Ho
2024-07-24 19:02 ` [PATCH mm-unstable v1 1/4] mm: don't hold css->refcnt during traversal Kinsey Ho
2024-07-25 16:33 ` kernel test robot
2024-07-25 20:43 ` Johannes Weiner [this message]
2024-07-25 21:09 ` Roman Gushchin
2024-07-25 22:33 ` Yosry Ahmed
2024-08-01 21:46 ` Kinsey Ho
2024-08-01 22:32 ` Kinsey Ho
2024-08-02 1:28 ` Johannes Weiner
2024-07-24 19:02 ` [PATCH mm-unstable v1 2/4] mm: increment gen # before restarting traversal Kinsey Ho
2024-07-25 20:53 ` Johannes Weiner
2024-07-24 19:02 ` [PATCH mm-unstable v1 3/4] mm: restart if multiple traversals raced Kinsey Ho
2024-07-24 19:02 ` [PATCH mm-unstable v1 4/4] mm: clean up mem_cgroup_iter() Kinsey Ho
2024-07-25 18:15 ` kernel test robot
2024-07-25 0:25 ` [PATCH mm-unstable v1 0/4] Improve mem_cgroup_iter() Roman Gushchin
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