linux-kernel - Re: [PATCH v2 -next] cgroup/pids: Avoid spurious event notification

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Message-ID: <555ce52c-ba32-37d9-13de-a05f773b21f2@huaweicloud.com>
Date: Wed, 31 Jul 2024 10:56:14 +0800
From: Xiu Jianfeng <xiujianfeng@...weicloud.com>
To: Tejun Heo <tj@...nel.org>
Cc: lizefan.x@...edance.com, hannes@...xchg.org, mkoutny@...e.com,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, xiujianfeng@...wei.com
Subject: Re: [PATCH v2 -next] cgroup/pids: Avoid spurious event notification



On 2024/7/31 6:14, Tejun Heo wrote:
> On Tue, Jul 30, 2024 at 03:29:20AM +0000, Xiu Jianfeng wrote:
>> From: Xiu Jianfeng <xiujianfeng@...wei.com>
>>
>> Currently when a process in a group forks and fails due to it's
>> parent's max restriction, all the cgroups from 'pids_forking' to root
>> will generate event notifications but only the cgroups from
>> 'pids_over_limit' to root will increase the counter of PIDCG_MAX.
>>
>> Consider this scenario: there are 4 groups A, B, C,and D, the
>> relationships are as follows, and user is watching on C.pids.events.
>>
>> root->A->B->C->D
>>
>> When a process in D forks and fails due to B.max restriction, the
>> user will get a spurious event notification because when he wakes up
>> and reads C.pids.events, he will find that the content has not changed.
>>
>> To address this issue, only the cgroups from 'pids_over_limit' to root
>> will have their PIDCG_MAX counters increased and event notifications
>> generated.
>>
>> Fixes: 385a635cacfe ("cgroup/pids: Make event counters hierarchical")
>> Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com>
> 
> Applied to cgroup/for-6.12.
> 
> Note that spurious events are explicitly allowed. Anyone watching an events
> file should keep track of the reported values to detect actual events.

Hi Tejun,

Thank you for your clarification. I thought it was not allowed.

Anyway, it's always good to avoid spurious events and disturbances to
users IMO.

> 
> Thanks.
>