Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756102Ab1BIXmz (ORCPT ); Wed, 9 Feb 2011 18:42:55 -0500 Received: from netrider.rowland.org ([192.131.102.5]:34852 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1755970Ab1BIXmp (ORCPT ); Wed, 9 Feb 2011 18:42:45 -0500 Date: Wed, 9 Feb 2011 18:42:43 -0500 (EST) From: Alan Stern X-X-Sender: stern@netrider.rowland.org To: Brendan Cully cc: Ian Campbell , "Rafael J. Wysocki" , , , "SUZUKI, Kazuhiro" , LKML Subject: Re: [linux-pm] [PATCH 0/2] Fix hangup after creating checkpoint on Xen. In-Reply-To: <20110209231659.GB249@zanzibar.kublai.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2689 Lines: 54 On Wed, 9 Feb 2011, Brendan Cully wrote: > > > That is odd, and it is quite different from the intended design of the > > > PM core. Drivers are supposed to put their devices into a known > > > suspended state; then afterwards they put the devices back into an > > > operational state. What happens while the devices are in the suspended > > > state isn't supposed to matter -- the system transition can fail, but > > > devices get treated exactly the same way as if it succeeded. > > > > > > Why do your drivers need to recover differently based on the success of > > > the hypercall? > > > > checkpointing isn't really my area but AIUI you don't want to do a full > > device teardown and reconnect like you would with a proper suspend > > because of the time that takes which prevents you from doing continuous > > rolling checkpoints at granularity which people want to implement > > various disaster recovery schemes. > > > > Hopefully one of the Xen checkpointing folks will chime in and explain > > why this is not possible to achieve at the individual driver level (or, > > even better, with a patch which does it that way ;-)). > > As Ian says, Xen has suspend_cancel because while the normal full > suspend/resume path works, it is much slower, and the work done during > resume is redundant. I don't remember the numbers offhand, but when we > added suspend_cancel I think we could do full suspend/resume in under > 100us, which was maybe a couple of orders of magnitude faster than > full resume (largely due to slow xenstore handshaking on resume, > IIRC). It made a big difference for our Remus HA project, which was > checkpointing tens of times per second. > > I'd like to keep the fast resume option, and expect that it can be > contained entirely in Xen-specific code. I'll try to get someone to > look into it here. > > I think fast resume is somewhat orthogonal to the problem of hanging > on resume, which just sounds like a xen-specific bug in the slow > path. In fact there already is a "fast suspend & resume" path in the PM core. It's the freeze/thaw procedure used when starting to hibernate. The documentation specifically says that drivers' freeze methods are supposed to quiesce their devices but not change power levels. In addition, the thaw method is invoked as part of recovery from a failed hibernation attempt, so it already has the "cancel" semantics that xen seems to want. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/