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SubjectRe: [PATCH-cgroup v2] cgroup: Move rcu_head up near the top of cgroup_root
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On 12/7/23 11:46, Michal Koutný wrote:
> On Thu, Dec 07, 2023 at 08:46:14AM -0500, Waiman Long <longman@redhat.com> wrote:
>> Commit 77070eeb8821 ("cgroup: Avoid false cacheline sharing of read
>> mostly rstat_cpu") happens to be the last straw that breaks it.
> FTR, when I build kernel from that commit, I can see
>
>> struct cgroup_root {
>> struct kernfs_root * kf_root; /* 0 8 */
>> unsigned int subsys_mask; /* 8 4 */
>> int hierarchy_id; /* 12 4 */
>>
>> /* XXX 48 bytes hole, try to pack */
>>
>> /* --- cacheline 1 boundary (64 bytes) --- */
>> struct cgroup cgrp __attribute__((__aligned__(64))); /* 64 2368 */
>>
>> /* XXX last struct has 8 bytes of padding */
>>
>> /* --- cacheline 38 boundary (2432 bytes) --- */
>> struct cgroup * cgrp_ancestor_storage; /* 2432 8 */
>> atomic_t nr_cgrps; /* 2440 4 */
>>
>> /* XXX 4 bytes hole, try to pack */
>>
>> struct list_head root_list; /* 2448 16 */
>> struct callback_head rcu __attribute__((__aligned__(8))); /* 2464 16 */
>> unsigned int flags; /* 2480 4 */
>> char release_agent_path[4096]; /* 2484 4096 */
>> /* --- cacheline 102 boundary (6528 bytes) was 52 bytes ago --- */
>> char name[64]; /* 6580 64 */
>>
>> /* size: 6656, cachelines: 104, members: 11 */
>> /* sum members: 6592, holes: 2, sum holes: 52 */
>> /* padding: 12 */
>> /* paddings: 1, sum paddings: 8 */
>> /* forced alignments: 2, forced holes: 1, sum forced holes: 48 */
>> } __attribute__((__aligned__(64)));
> 2480 has still quite a reserve below 4096. (I can't see an CONFIG_*
> affecting this.)
>
> Perhaps, I missed something from the linux-next merging thread?

CONFIG_LOCKDEP and some other debug configs are enabled with
allmodconfig. This can greatly increase the size of some of the
structures. I am not able to use pahole due to missing BTF info so I
don't the exact size. However, I can reproduce the build failure and the
patch is  able to fix it.

Cheers,
Longman

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