linux-kernel - Re: [PATCH v3 4/9] cgroup/cpuset: Enable event notification when partition become invalid

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Date:   Tue, 27 Jul 2021 16:46:18 -0400
From:   Waiman Long <llong@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Koutný <mkoutny@...e.com>
Subject: Re: [PATCH v3 4/9] cgroup/cpuset: Enable event notification when
 partition become invalid

On 7/27/21 4:26 PM, Waiman Long wrote:
> On 7/26/21 7:14 PM, Tejun Heo wrote:
>> On Tue, Jul 20, 2021 at 10:18:29AM -0400, Waiman Long wrote:
>>> +static inline void notify_partition_change(struct cpuset *cs,
>>> +                       int old_prs, int new_prs)
>>> +{
>>> +    if ((old_prs == new_prs) ||
>>> +       ((old_prs != PRS_ERROR) && (new_prs != PRS_ERROR)))
>>> +        return;
>>> +    cgroup_file_notify(&cs->partition_file);
>> I'd generate an event on any state changes. The user have to read the 
>> file
>> to find out what happened anyway.
>>
>> Thanks.
>
> From my own testing with "inotify_add_watch(fd, file, IN_MODIFY)", 
> poll() will return with a event whenever a user write to 
> cpuset.cpus.partition control file. I haven't really look into the 
> sysfs code yet, but I believe event generation will be automatic in 
> this case. So I don't think I need to explicitly add a 
> cgroup_file_notify() when users modify the control file directly. 
> Other indirect modification may cause the partition value to change 
> to/from PRS_ERROR and I should have captured all those changes in this 
> patchset. I will update the patch to note this point to make it more 
> clear. 

After thinking about it a bit more it, it is probably not a problem to 
call cgroup_file_notify() for every change as this is not in a 
performance critical path anyway. I will do some more testing to find 
out if doing cgroup_file_notify() for regular file write will cause an 
extra duplicated event to be sent out, I will probably stay with the 
current patch. Otherwise, I can change it to always call 
cgroup_file_notify().

Cheers,
Longman