Re: [PATCH RESEND 1/1] x86: tsc: avoid system instability in hibernation

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Jul 26, 2018 at 5:56 PM, Eduardo Valentin <eduval@amazon.com> wrote:
> 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,

Isn't that the actual bug?

> 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.
>
> Situation like this can be reported by using lockup detectors
> such as workqueue lockup detectors:
>
> [root@ip-172-31-67-114 ec2-user]# echo disk > /sys/power/state
>
> Message from syslogd@ip-172-31-67-114 at May  7 18:23:21 ...
>  kernel:BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 57s!
>
> Message from syslogd@ip-172-31-67-114 at May  7 18:23:21 ...
>  kernel:BUG: workqueue lockup - pool cpus=1 node=0 flags=0x0 nice=0 stuck for 57s!
>
> Message from syslogd@ip-172-31-67-114 at May  7 18:23:21 ...
>  kernel:BUG: workqueue lockup - pool cpus=3 node=0 flags=0x1 nice=0 stuck for 57s!
>
> Message from syslogd@ip-172-31-67-114 at May  7 18:29:06 ...
>  kernel:BUG: workqueue lockup - pool cpus=3 node=0 flags=0x1 nice=0 stuck for 403s!
>
> 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.

I would rather call it a workaround.

> 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.
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Dou Liyang <douly.fnst@cn.fujitsu.com>
> Cc: Len Brown <len.brown@intel.com>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
> Cc: Eduardo Valentin <eduval@amazon.com>
> Cc: "mike.travis@hpe.com" <mike.travis@hpe.com>
> Cc: Rajvi Jingar <rajvi.jingar@intel.com>
> Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
> Cc: Philippe Ombredanne <pombredanne@nexb.com>
> Cc: Kate Stewart <kstewart@linuxfoundation.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-pm@vger.kernel.org
> Signed-off-by: Eduardo Valentin <eduval@amazon.com>
> ---
>
> Hey,
>
> No changes from first attempt, no pressure on resending. The RESEND
> tag is just because I missed linux-pm in the first attempt.
>
> BR,
>
>  arch/x86/kernel/tsc.c       | 29 +++++++++++++++++++++++++++++
>  include/linux/sched/clock.h |  5 +++++
>  kernel/sched/clock.c        |  4 ++--
>  3 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 8ea117f8142e..f197c9742fef 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -13,6 +13,7 @@
>  #include <linux/percpu.h>
>  #include <linux/timex.h>
>  #include <linux/static_key.h>
> +#include <linux/suspend.h>
>
>  #include <asm/hpet.h>
>  #include <asm/timer.h>
> @@ -1377,3 +1378,31 @@ unsigned long calibrate_delay_is_known(void)
>         return 0;
>  }
>  #endif
> +
> +static int tsc_pm_notifier(struct notifier_block *notifier,
> +                          unsigned long pm_event, void *unused)
> +{
> +       switch (pm_event) {
> +       case PM_HIBERNATION_PREPARE:
> +               clear_sched_clock_stable();
> +               break;

This is too early IMO.  This happens before hibernation starts, even
before the image is created.

> +       case PM_POST_HIBERNATION:
> +               /* Set back to the default */
> +               if (!check_tsc_unstable())
> +                       set_sched_clock_stable();
> +               break;
> +       }
> +
> +       return 0;
> +};

If anything like this is the way to go, which honestly I doubt, I
would prefer it to be done in hibernate() in the !in_suspend case.

But why does it only affect hibernation?  Do we do something extra for
system-wide suspend/resume that is not done for hibernation?