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Date:   Fri, 23 Sep 2022 09:29:03 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Zhongkun He <hezhongkun.hzk@bytedance.com>
Cc:     hannes@cmpxchg.org, roman.gushchin@linux.dev,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org, lizefan.x@bytedance.com,
        wuyun.abel@bytedance.com
Subject: Re: [External] Re: [PATCH] cgroup/cpuset: Add a new isolated
 mems.policy type.
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On Wed 14-09-22 23:10:47, Zhongkun He wrote:
> > > 
> > > > Back to the previous question.
> > > > > The question is how to implement that with a sensible semantic.
> > > > 
> > > > Thanks for your analysis and suggestions.It is really difficult to add
> > > > policy directly to cgroup for the hierarchical enforcement. It
> > > > would be a good idea to add pidfd_set_mempolicy.
> > > 
> > > Are you going to pursue that path?
> 
> > Hi Michal, thanks for your suggestion and reply.
> > 
> >  > Are you going to pursue that path?
> > 
> > Yes，I'll give it a try as it makes sense to modify the policy dynamically.
> > 
> > Thanks.
> 
> Hi Michal, i have a question about pidfd_set_mempolicy, it would be better
> if you have some suggestions.
> 
> The task_struct of processes and threads are independent. If we change the
> mempolicy of the process through pidfd_set_mempolicy, the mempolicy of its
> thread will not change. Of course users can set the mempolicy of all threads
> by iterating through /proc/tgid/task.
> 
> The question is whether we should override the thread's mempolicy when
> setting the process's mempolicy.
> 
> There are two options:
> A:Change the process's mempolicy and set that mempolicy to all it's threads.
> B:Only change the process's mempolicy in kernel. The mempolicy of the thread
> needs to be modified by the user through pidfd_set_mempolicy in
> userspace, if necessary.

set_mempolicy is a per task_struct operation and so should be pidfd
based API as well. If somebody requires a per-thread-group setting then
the whole group should be iterated. I do not think we have any
precendence where pidfd operation on the thread group leader has side
effects on other threads as well.
-- 
Michal Hocko
SUSE Labs