Re: [RFC] Shared page accounting for memory cgroup

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-01-04 09:35:28]:

> On Mon, 4 Jan 2010 05:37:52 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-01-04 08:51:08]:
> > 
> > > On Tue, 29 Dec 2009 23:57:43 +0530
> > > Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> > > 
> > > > Hi, Everyone,
> > > > 
> > > > I've been working on heuristics for shared page accounting for the
> > > > memory cgroup. I've tested the patches by creating multiple cgroups
> > > > and running programs that share memory and observed the output.
> > > > 
> > > > Comments?
> > > 
> > > Hmm? Why we have to do this in the kernel ?
> > >
> > 
> > For several reasons that I can think of
> > 
> > 1. With task migration changes coming in, getting consistent data free of races
> > is going to be hard.
> 
> Hmm, Let's see real-worlds's "ps" or "top" command. Even when there are no guarantee
> of error range of data, it's still useful.

Yes, my concern is this

1. I iterate through tasks and calculate RSS
2. I look at memory.usage_in_bytes

If the time in user space between 1 and 2 is large I get very wrong
results, specifically if the workload is changing its memory usage
drastically.. no?

> 
> > 2. The cost of doing it in the kernel is not high, it does not impact
> > the memcg runtime, it is a request-response sort of cost.
> >
> > 3. The cost in user space is going to be high and the implementation
> > cumbersome to get right.
> >  
> I don't like moving a cost in the userland to the kernel.

Me neither, but I don't think it is a fixed overhead.

 Considering 
> real-time kernel or full-preemptive kernel, this very long read_lock() in the
> kernel is not good, IMHO. (I think css_set_lock should be mutex/rw-sem...)

I agree, we should discuss converting the lock to a mutex or a
semaphore, but there might be a good reason for keeping it as a
spin_lock.

> cgroup_iter_xxx can block cgroup_post_fork() and this may cause critical
> system delay of milli-seconds.
> 

Agreed, but then that can happen, even while attaching a task, seeing
cgroup tasks file (list of tasks).

> BTW, if you really want to calculate somthing in atomic, I think following
> interface may be welcomed for freezing.
> 
>   cgroup.lock
>   # echo 1 > /...../cgroup.lock 
>     All task move, mkdir, rmdir to this cgroup will be blocked by mutex.
>     (But fork/exit will not be blocked.)
> 
>   # echo 0 > /...../cgroup.lock
>     Unlock.
> 
>   # cat /...../cgroup.lock
>     show lock status and lock history (for debug).
> 
> Maybe good for some kinds of middleware.
> But this may be difficult if we have to consider hierarchy.
>

I don't like the idea of providing an interface that can control
kernel locks from user space, user space can tangle up and get it
wrong. 

-- 
	Balbir
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