Re: [PATCH v3 1/1] kthread: allocate kthread structure using kmalloc

From: Roman Penyaev <roman.penyaev@profitbricks.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/1] kthread: allocate kthread structure using kmalloc
Date: Tue, 25 Oct 2016 18:08:31 +0200	[thread overview]
Message-ID: <CAJrWOzArjn5-byFx1y9VLT9phnQBTQpWAhX96RVigRsF0h2pOQ@mail.gmail.com> (raw)
In-Reply-To: <20161025154301.GA12015@redhat.com>

On Tue, Oct 25, 2016 at 5:43 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 10/25, Oleg Nesterov wrote:
>>
>> On 10/25, Roman Pen wrote:
>> >
>> > This patch avoids allocation of kthread structure on a stack, and simply
>> > uses kmalloc.
>>
>> Oh. I didn't even read this patch, but I have to admit I personally do not
>> like it. I can be wrong, but imo this is the step to the wrong direction.
>
> And after I tried to actually read it I dislike it even more, sorry Roman.
> Starting from the fact it moves kthread_create_info into struct kthread.

that can be changed, of course, as I told, I wanted to keep allocations/
deallocations simpler.

>> struct kthread is already bloated, we should not bloat it more. Instead
>> we should kill it. And to_kthread() too, at least in its current form.
>
> Yes, but even if we can't or do not want to do this, even if we want to
> kmalloc struct kthread, I really think it should not be refcounted
> separately from task_struct.

it is already like that, we have to get/put references on a task stack.

>
> something like the patch in http://marc.info/?l=linux-kernel&m=146715459127804

the key function in that patch is:

free_kthread_struct(tsk);

so if we teach the generic free_task() to deal with kthreads, that of course
solves these kind of problems.  I did not consider that variant.

>
> Either way to_live_kthread() must go away. Currently we can't avoid it
> because we abuse vfork_done, but as I already said we no longer need this.

There is something which I do not understand.  You still need to have a
connection (a pointer) between task_struct and private data (kthread AND
private data, whatever), which is passed by the user of kthread API.
You still need to find a victim in a task_struct and abuse it :)

So in particular I do not understand this comment from the patch above
where you abuse 'current->set_child_tid':

 * This is the ugly but simple hack we will hopefully remove soon.

how you are going to avoid this abuse of set_child_tid? or vfork_done?
because vfork_done is not only for waking up (yes, I totally agree, we
can reuse task_work), it is also for getting a private data (like
workqueue uses it):  task_struct->vfork_done->kthread->data.

--
Roman