Date: Wed, 6 Mar 2013 11:31:20 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Stephane Eranian <eranian@google.com>
Cc: Li Zefan <lizefan@huawei.com>, LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] perf: remove include of cgroup.h from perf_event.h
Message-ID: <20130306103120.GA16827@gmail.com>
References: <513568A0.6020804@huawei.com>
 <CABPqkBTTn6E=FS6WiCWopcScHDVa9F6bu5-pw8Ma7TK2aunkvQ@mail.gmail.com>
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* Stephane Eranian <eranian@google.com> wrote:

> > - * This is a per-cpu dynamically allocated data structure.
> > - */
> > -struct perf_cgroup_info {
> > -       u64                             time;
> > -       u64                             timestamp;
> > -};
> > -
> > -struct perf_cgroup {
> > -       struct                          cgroup_subsys_state css;
> > -       struct                          perf_cgroup_info *info; /* timing info, one per cpu */
> > -};
> > -#endif
> > -
> > +struct perf_cgroup;
> 
> The problem is that you have struct perf_cgroup in the struct perf_event 
> structure. Today, this field is not referenced outside of kernel/events/core.c But 
> it is available outside this file. If someday the field is reference, your changes 
> will have to do reverted. So I am wondering what is the point of the change right 
> now?

It's standard practice to not define the type for task_struct or other kernel 
subsystems.

For example slab caches can be created via kmem_cache_create() anywhere in the tree, 
but the internal structure is only known to the SLAB subsystem.

Thanks,

	Ingo
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