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 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

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.

Balbir Singh.

> > > > Reviewed-by: Erik Quanstrom <quanstro@xxxxxxxxxx>
> > > > Reviewed-by: Frank van der Linden <fllinden@xxxxxxxxxx>
> > > > Reviewed-by: Balbir Singh <sblbir@xxxxxxxxxx>
> > > > Reviewed-by: Munehisa Kamata <kamatam@xxxxxxxxxx>
> > > > Tested-by: Anchal Agarwal <anchalag@xxxxxxxxxx>
> > > > Signed-off-by: Eduardo Valentin <eduval@xxxxxxxxxx>
> > > > ---
> > > 
> > > NAK, the code very much relies on never getting marked stable again
> > > after it gets set to unstable.
> > > 
> > 
> > Well actually, at the PM_POST_HIBERNATION, we do the check and set stable
> > if
> > known to be stable.
> > 
> > The issue only really happens during the restoration path under scheduling
> > pressure,
> > which takes forever to finish, as described in the commit.
> > 
> > Do you see a better solution for this issue?
> 
> I still have no clue what your actual problem is. You say scheduling
> goes wobbly because sched_clock_data is stale, but when stable that
> doesn't matter.
> 
> So what is the actual problem?
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