Re: [Xen-devel] [RFC PATCH V2 11/11] x86: tsc: avoid system instability in hibernation

["Singh, Balbir" <sblbir@xxxxxxxxxx>]

On Mon, 2020-01-13 at 13:01 +0000, Andrew Cooper wrote:
> On 13/01/2020 11:43, Singh, Balbir wrote:
> > On Mon, 2020-01-13 at 11:16 +0100, Peter Zijlstra wrote:
> > > On Fri, Jan 10, 2020 at 07:35:20AM -0800, Eduardo Valentin wrote:
> > > > Hey Peter,
> > > > 
> > > > On Wed, Jan 08, 2020 at 11:50:11AM +0100, Peter Zijlstra wrote:
> > > > > On Tue, Jan 07, 2020 at 11:45:26PM +0000, Anchal Agarwal wrote:
> > > > > > From: Eduardo Valentin <eduval@xxxxxxxxxx>
> > > > > > 
> > > > > > System instability are seen during resume from hibernation when
> > > > > > system
> > > > > > is under heavy CPU load. This is due to the lack of update of
> > > > > > sched
> > > > > > clock data, and the scheduler would then think that heavy CPU hog
> > > > > > tasks need more time in CPU, causing the system to freeze
> > > > > > during the unfreezing of tasks. For example, threaded irqs,
> > > > > > and kernel processes servicing network interface may be delayed
> > > > > > for several tens of seconds, causing the system to be unreachable.
> > > > > > The fix for this situation is to mark the sched clock as unstable
> > > > > > as early as possible in the resume path, leaving it unstable
> > > > > > for the duration of the resume process. This will force the
> > > > > > scheduler to attempt to align the sched clock across CPUs using
> > > > > > the delta with time of day, updating sched clock data. In a post
> > > > > > hibernation event, we can then mark the sched clock as stable
> > > > > > again, avoiding unnecessary syncs with time of day on systems
> > > > > > in which TSC is reliable.
> > > > > 
> > > > > This makes no frigging sense what so bloody ever. If the clock is
> > > > > stable, we don't care about sched_clock_data. When it is stable you
> > > > > get
> > > > > a linear function of the TSC without complicated bits on.
> > > > > 
> > > > > When it is unstable, only then do we care about the
> > > > > sched_clock_data.
> > > > > 
> > > > 
> > > > Yeah, maybe what is not clear here is that we covering for situation
> > > > where clock stability changes over time, e.g. at regular boot clock is
> > > > stable, hibernation happens, then restore happens in a non-stable
> > > > clock.
> > > 
> > > Still confused, who marks the thing unstable? The patch seems to suggest
> > > you do yourself, but it is not at all clear why.
> > > 
> > > If TSC really is unstable, then it needs to remain unstable. If the TSC
> > > really is stable then there is no point in marking is unstable.
> > > 
> > > Either way something is off, and you're not telling me what.
> > > 
> > 
> > Hi, Peter
> > 
> > For your original comment, just wanted to clarify the following:
> > 
> > 1. After hibernation, the machine can be resumed on a different but
> > compatible
> > host (these are VM images hibernated)
> > 2. This means the clock between host1 and host2 can/will be different
> 
> The guests TSC value is part of all save/migrate/resume state.  Given
> this bug, I presume you've actually discarded all register state on
> hibernate, and the TSC is starting again from 0?
> 
> The frequency of the new TSC might very likely be different, but the
> scale/offset in the paravirtual clock information should let Linux's
> view of time stay consistent.
> 

I am looking at my old dmesg logs, which I seem to have lost to revalidate,
but I think Eduardo had a different point. I should point out that I was
adding to the list of potentially missed assumptions


> > In your comments are you making the assumption that the host(s) is/are the
> > same? Just checking the assumptions being made and being on the same page
> > with
> > them.
> 
> TSCs are a massive source of "fun".  I'm not surprised that there are
> yet more bugs around.
> 
> Does anyone actually know what does/should happen to the real TSC on
> native S4?  The default course of action should be for virtualisation to
> follow suit.
> 
> ~Andrew

Balbir
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