From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oren Laadan Subject: Re: [C/R v20][PATCH 15/96] cgroup freezer: Fix buggy resume test for tasks frozen with cgroup freezer Date: Tue, 23 Mar 2010 12:03:37 -0400 Message-ID: <4BA8E659.1030702@cs.columbia.edu> References: <1268842164-5590-1-git-send-email-orenl@cs.columbia.edu> <1268842164-5590-15-git-send-email-orenl@cs.columbia.edu> <1268842164-5590-16-git-send-email-orenl@cs.columbia.edu> <201003230028.40915.rjw@sisk.pl> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-2; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <201003230028.40915.rjw@sisk.pl> Sender: owner-linux-mm@kvack.org To: "Rafael J. Wysocki" Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Serge Hallyn , Ingo Molnar , containers@lists.linux-foundation.org, Matt Helsley , Cedric Le Goater , Paul Menage , Li Zefan , Pavel Machek , linux-pm@lists.linux-foundation.org List-Id: linux-api@vger.kernel.org Rafael J. Wysocki wrote: > On Wednesday 17 March 2010, Oren Laadan wrote: >> From: Matt Helsley >> >> When the cgroup freezer is used to freeze tasks we do not want to thaw >> those tasks during resume. Currently we test the cgroup freezer >> state of the resuming tasks to see if the cgroup is FROZEN. If so >> then we don't thaw the task. However, the FREEZING state also indicates >> that the task should remain frozen. >> >> This also avoids a problem pointed out by Oren Ladaan: the freezer state >> transition from FREEZING to FROZEN is updated lazily when userspace reads >> or writes the freezer.state file in the cgroup filesystem. This means that >> resume will thaw tasks in cgroups which should be in the FROZEN state if >> there is no read/write of the freezer.state file to trigger this >> transition before suspend. >> >> NOTE: Another "simple" solution would be to always update the cgroup >> freezer state during resume. However it's a bad choice for several reasons: >> Updating the cgroup freezer state is somewhat expensive because it requires >> walking all the tasks in the cgroup and checking if they are each frozen. >> Worse, this could easily make resume run in N^2 time where N is the number >> of tasks in the cgroup. Finally, updating the freezer state from this code >> path requires trickier locking because of the way locks must be ordered. >> >> Instead of updating the freezer state we rely on the fact that lazy >> updates only manage the transition from FREEZING to FROZEN. We know that >> a cgroup with the FREEZING state may actually be FROZEN so test for that >> state too. This makes sense in the resume path even for partially-frozen >> cgroups -- those that really are FREEZING but not FROZEN. >> >> Reported-by: Oren Ladaan >> Signed-off-by: Matt Helsley >> Cc: Cedric Le Goater >> Cc: Paul Menage >> Cc: Li Zefan >> Cc: Rafael J. Wysocki >> Cc: Pavel Machek >> Cc: linux-pm@lists.linux-foundation.org > > Looks reasonable. > > Is anyone handling that already or do you want me to take it to my tree? Yes, please do. Thanks ! Oren. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org