From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751498Ab3BTVgp (ORCPT ); Wed, 20 Feb 2013 16:36:45 -0500 Received: from out02.mta.xmission.com ([166.70.13.232]:52148 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751278Ab3BTVgn (ORCPT ); Wed, 20 Feb 2013 16:36:43 -0500 From: ebiederm@xmission.com (Eric W. Biederman) To: Alan Stern Cc: Li Fei , , , , , , , , , , , , , References: Date: Wed, 20 Feb 2013 13:36:19 -0800 In-Reply-To: (Alan Stern's message of "Wed, 20 Feb 2013 15:48:43 -0500 (EST)") Message-ID: <87zjyyekh8.fsf@xmission.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-AID: U2FsdGVkX19KB1V0cIRXFAcBe3Xf56xHfBWuCKK29Po= X-SA-Exim-Connect-IP: 98.207.153.68 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-Report: * -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP * 0.1 XMSubLong Long Subject * 0.0 T_TM2_M_HEADER_IN_MSG BODY: T_TM2_M_HEADER_IN_MSG * -3.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% * [score: 0.0083] * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa06 1397; Body=1 Fuz1=1 Fuz2=1] * 0.0 T_XMDrugObfuBody_14 obfuscated drug references X-Spam-DCC: XMission; sa06 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: ;Alan Stern X-Spam-Relay-Country: Subject: Re: [PATCH] freezer: configure user space process frozen along with kernel threads X-Spam-Flag: No X-SA-Exim-Version: 4.2.1 (built Wed, 14 Nov 2012 14:26:46 -0700) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Alan Stern writes: > On Wed, 20 Feb 2013, Eric W. Biederman wrote: > >> >> Why can't the fuse filesystem freeze when there are requests pending? >> > >> > It _can_ freeze (that is, the fuse daemon can). The problem is that >> > tasks _using_ the fuse filsystem can't if the daemon doesn't respond. >> >> Which is what I meant when I said that the fuse filesystem couldn't >> freeze. > > Oh, okay. But it's no different from any other filesystem in that > respect. Processes generally can't be frozen while they are waiting > for filesystem I/O to complete. In many cases they can't receive > signals either (they are in an uninterruptible wait state). Ick. So the process freezer and all network filesystems has problems? Especially nfs? >> > These tasks are stuck in uninterruptible wait states deep in the >> > filesystem layer, probably holding important locks. They can't be >> > frozen until the outstanding requests complete. >> >> Why is it that processes that can be preempted can't be frozen? > > There's a big difference between preemption and freezing: Preemption > is involuntary whereas freezing is voluntary. It's like the difference > between preemptive and cooperative multitasking. I hadn't realized freezing was voluntary. That certainly seems like a pain. > Processes can be frozen only by making explicit checks, and they > mustn't be frozen while they are holding locks that would prevent other > processes from reaching one of those checks. > >> At most I would suggest that processes be frozen in reverse priority >> order. Which unless there is a priority inversion should solve this >> problem without an additional proc file. > > Do fuse daemons (and the processes they rely upon) run with elevated > priority? I don't know if the daemons are of an elevated scheduling priority today but if they aren't it is as easy to require an elevated scheduling priority as it is to require a magic freezer priority. Furthermore if they don't run at an elevated priority there is the possibility of priority inversion. With a little care you might even be able to drop the kernel thread special case if you freeze processes by prirority. Eric