Date:   Wed, 22 Jan 2020 20:07:10 +0000
From:   Anchal Agarwal <anchalag@...zon.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        "Singh, Balbir" <sblbir@...zon.com>,
        "Valentin, Eduardo" <eduval@...zon.com>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
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Subject: Re: [RFC PATCH V2 11/11] x86: tsc: avoid system instability in
 hibernation

On Tue, Jan 14, 2020 at 07:29:52PM +0000, Anchal Agarwal wrote:
> On Tue, Jan 14, 2020 at 12:30:02AM +0100, Rafael J. Wysocki wrote:
> > On Mon, Jan 13, 2020 at 10:50 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > >
> > > On Mon, Jan 13, 2020 at 1:43 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > > >
> > > > On Mon, Jan 13, 2020 at 11:43:18AM +0000, Singh, Balbir wrote:
> > > > > 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.
> > > >
> > > > I would expect this to be the same problem we have as regular suspend,
> > > > after power off the TSC will have been reset, so resume will have to
> > > > somehow bridge that gap. I've no idea if/how it does that.
> > >
> > > In general, this is done by timekeeping_resume() and the only special
> > > thing done for the TSC appears to be the tsc_verify_tsc_adjust(true)
> > > call in tsc_resume().
> > 
> > And I forgot about tsc_restore_sched_clock_state() that gets called
> > via restore_processor_state() on x86, before calling
> > timekeeping_resume().
> >
> In this case tsc_verify_tsc_adjust(true) this does nothing as
> feature bit X86_FEATURE_TSC_ADJUST is not available to guest. 
> I am no expert in this area, but could this be messing things up?
> 
> Thanks,
> Anchal
Gentle nudge on this. I will add more data here in case that helps.

1. Before this patch, tsc is stable but hibernation does not work
100% of the time. I agree if tsc is stable it should not be marked
unstable however, in this case if I run a cpu intensive workload
in the background and trigger reboot-hibernation loop I see a 
workqueue lockup. 

2. The lockup does not hose the system completely,
the reboot-hibernation carries out and system recovers. 
However, as mentioned in the commit message system does 
become unreachable for couple of seconds.

3. Xen suspend/resume seems to save/restore time_memory area in its
xen_arch_pre_suspend and xen_arch_post_suspend. The xen clock value
is saved. xen_sched_clock_offset is set at resume time to ensure a
monotonic clock value

4. Also, the instances do not have InvariantTSC exposed. Feature bit
X86_FEATURE_TSC_ADJUST is not available to guest and xen clocksource
is used by guests.

I am not sure if something needs to be fixed on hibernate path itself
or its very much ties to time handling on xen guest hibernation

Here is a part of log from last hibernation exit to next hibernation
entry. The loop was running for a while so boot to lockup log will be
huge. I am specifically including the timestamps.

...
01h 57m 15.627s(  16ms): [    5.822701] OOM killer enabled.
01h 57m 15.627s(   0ms): [    5.824981] Restarting tasks ... done.
01h 57m 15.627s(   0ms): [    5.836397] PM: hibernation exit
01h 57m 17.636s(2009ms): [    7.844471] PM: hibernation entry
01h 57m 52.725s(35089ms): [   42.934542] BUG: workqueue lockup - pool cpus=0
node=0 flags=0x0 nice=0 stuck for 37s!
01h 57m 52.730s(   5ms): [   42.941468] Showing busy workqueues and worker
pools:
01h 57m 52.734s(   4ms): [   42.945088] workqueue events: flags=0x0
01h 57m 52.737s(   3ms): [   42.948385]   pwq 0: cpus=0 node=0 flags=0x0 nice=0
active=2/256
01h 57m 52.742s(   5ms): [   42.952838]     pending: vmstat_shepherd,
check_corruption
01h 57m 52.746s(   4ms): [   42.956927] workqueue events_power_efficient:
flags=0x80
01h 57m 52.749s(   3ms): [   42.960731]   pwq 0: cpus=0 node=0 flags=0x0 nice=0
active=4/256
01h 57m 52.754s(   5ms): [   42.964835]     pending: neigh_periodic_work,
do_cache_clean [sunrpc], neigh_periodic_work, check_lifetime
01h 57m 52.781s(  27ms): [   42.971419] workqueue mm_percpu_wq: flags=0x8
01h 57m 52.781s(   0ms): [   42.974628]   pwq 0: cpus=0 node=0 flags=0x0 nice=0
active=1/256
01h 57m 52.781s(   0ms): [   42.978901]     pending: vmstat_update
01h 57m 52.781s(   0ms): [   42.981822] workqueue ipv6_addrconf: flags=0x40008
01h 57m 52.781s(   0ms): [   42.985524]   pwq 0: cpus=0 node=0 flags=0x0 nice=0
active=1/1
01h 57m 52.781s(   0ms): [   42.989670]     pending: addrconf_verify_work [ipv6]
01h 57m 52.782s(   1ms): [   42.993282] workqueue xfs-conv/xvda1: flags=0xc
01h 57m 52.786s(   4ms): [   42.996708]   pwq 0: cpus=0 node=0 flags=0x0 nice=0
active=3/256
01h 57m 52.790s(   4ms): [   43.000954]     pending: xfs_end_io [xfs],
xfs_end_io [xfs], xfs_end_io [xfs]
01h 57m 52.795s(   5ms): [   43.005610] workqueue xfs-reclaim/xvda1: flags=0xc
01h 57m 52.798s(   3ms): [   43.008945]   pwq 0: cpus=0 node=0 flags=0x0 nice=0
active=1/256
01h 57m 52.802s(   4ms): [   43.012675]     pending: xfs_reclaim_worker [xfs]
01h 57m 52.805s(   3ms): [   43.015741] workqueue xfs-sync/xvda1: flags=0x4
01h 57m 52.808s(   3ms): [   43.018723]   pwq 0: cpus=0 node=0 flags=0x0 nice=0
active=1/256
01h 57m 52.811s(   3ms): [   43.022436]     pending: xfs_log_worker [xfs]
01h 57m 52.814s(   3ms): [   43.043519] Filesystems sync: 35.234 seconds
01h 57m 52.837s(  23ms): [   43.048133] Freezing user space processes ...
(elapsed 0.001 seconds) done.
01h 57m 52.844s(   7ms): [   43.055996] OOM killer disabled.
01h 57m 53.838s( 994ms): [   43.061512] PM: Preallocating image memory... done
(allocated 385859 pages)
01h 57m 53.843s(   5ms): [   44.054720] PM: Allocated 1543436 kbytes in 1.06
seconds (1456.07 MB/s)
01h 57m 53.861s(  18ms): [   44.060885] Freezing remaining freezable tasks ...
(elapsed 0.001 seconds) done.
01h 57m 53.861s(   0ms): [   44.069715] printk: Suspending console(s) (use
no_console_suspend to debug)
01h 57m 56.278s(2417ms): [   44.116601] Disabling non-boot CPUs ...
.....
hibernate-resume loop continues after this. As mentioned above, I loose
connectivity for a while.


Thanks,
Anchal

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