From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Blum Subject: Re: [PATCH v8 0/3] cgroups: implement moving a threadgroup's threads atomically with cgroup.procs Date: Wed, 9 Feb 2011 20:36:06 -0500 Message-ID: <20110210013606.GA26677__39121.3730596521$1297301824$gmane$org@ghc17.ghc.andrew.cmu.edu> References: <20101226120919.GA28529@ghc17.ghc.andrew.cmu.edu> <20110208013542.GC31569@ghc17.ghc.andrew.cmu.edu> <20110209151046.89e03dcd.akpm@linux-foundation.org> <20110210100210.adf09c49.kamezawa.hiroyu@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20110210100210.adf09c49.kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: KAMEZAWA Hiroyuki Cc: Ben Blum , containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org, Miao Xie , David Rientjes , Andrew Morton , menage-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org List-Id: containers.vger.kernel.org On Thu, Feb 10, 2011 at 10:02:10AM +0900, KAMEZAWA Hiroyuki wrote: > On Wed, 9 Feb 2011 15:10:46 -0800 > Andrew Morton wrote: > > > On Mon, 7 Feb 2011 20:35:42 -0500 > > Ben Blum wrote: > > > > > On Sun, Dec 26, 2010 at 07:09:19AM -0500, Ben Blum wrote: > > > > On Fri, Dec 24, 2010 at 03:22:26AM -0500, Ben Blum wrote: > > > > > On Wed, Aug 11, 2010 at 01:46:04AM -0400, Ben Blum wrote: > > > > > > On Fri, Jul 30, 2010 at 07:56:49PM -0400, Ben Blum wrote: > > > > > > > This patch series is a revision of http://lkml.org/lkml/2010/6/25/11 . > > > > > > > > > > > > > > This patch series implements a write function for the 'cgroup.procs' > > > > > > > per-cgroup file, which enables atomic movement of multithreaded > > > > > > > applications between cgroups. Writing the thread-ID of any thread in a > > > > > > > threadgroup to a cgroup's procs file causes all threads in the group to > > > > > > > be moved to that cgroup safely with respect to threads forking/exiting. > > > > > > > (Possible usage scenario: If running a multithreaded build system that > > > > > > > sucks up system resources, this lets you restrict it all at once into a > > > > > > > new cgroup to keep it under control.) > > > > > > > > > > > > > > Example: Suppose pid 31337 clones new threads 31338 and 31339. > > > > > > > > > > > > > > # cat /dev/cgroup/tasks > > > > > > > ... > > > > > > > 31337 > > > > > > > 31338 > > > > > > > 31339 > > > > > > > # mkdir /dev/cgroup/foo > > > > > > > # echo 31337 > /dev/cgroup/foo/cgroup.procs > > > > > > > # cat /dev/cgroup/foo/tasks > > > > > > > 31337 > > > > > > > 31338 > > > > > > > 31339 > > > > > > > > > > > > > > A new lock, called threadgroup_fork_lock and living in signal_struct, is > > > > > > > introduced to ensure atomicity when moving threads between cgroups. It's > > > > > > > taken for writing during the operation, and taking for reading in fork() > > > > > > > around the calls to cgroup_fork() and cgroup_post_fork(). > > > > The above six month old text is the best (and almost the only) > > explanation of the rationale for the entire patch series. Is > > it still correct and complete? Yep, it's still fresh. (That's why I kept it around!) > > > > > > Assuming "yes", then... how do we determine whether the feature is > > sufficiently useful to justify merging and maintaining it? Will people > > actually use it? > > > > Was there some particular operational situation which led you to think > > that the kernel should have this capability? If so, please help us out here > > and lavishly describe it. > > > > In these months, I saw following questions as > == > Q. I think I put qemu to xxxx cgroup but it never works! > A. You need to put all threads in qemu to cgroup. > == > > 'tasks' file is not useful interface for users, I think. > (Even if users tend to use put-task-before-exec scheme.) > > > IMHO, from user's side of view, 'tasks' file is a mystery. > > TID(thread-ID) is one of secrets in Linux + pthread library. For example, > on RHEL6, to use gettid(), users has to use syscall() directly. And end-user > may not know about thread-ID which is hidden under pthreads. I think glibc in general is to blame for the fact that you need to syscall(__NR_gettid)? Regardless - yes, exposing an interface dealing with task_structs can be less than perfect for a world that deals in userland applications. > IIRC, there are no interface other than /proc//tasks which shows all > thread IDs of a process. But it's not atomic. I tend to use pgrep, which is a bit of a hassle. Also, like in the six-month-old-text, many resource-sucking programs nowadays (web browsers) are multithreaded. > So, I think it's ok to have 'procs' interface for cgroup if > overhead/impact of patch is not heavy. > > Thanks, > -Kame Thanks for the reasoning. ;) -- Ben