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Date: Tue, 12 May 2015 19:17:24 -0400
From: Sasha Levin <sasha.levin@oracle.com>
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To: KVM General <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
CC: mtosatti@redhat.com, Paolo Bonzini <pbonzini@redhat.com>,
        lcapitulino@redhat.com
Subject: kvm: odd time values since "kvmclock: set scheduler clock stable"
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Hi all,

I'm seeing odd jump in time values during boot of a KVM guest:

[...]
[    0.000000] tsc: Detected 2260.998 MHz processor
[3376355.247558] Calibrating delay loop (skipped) preset value..
[...]

I've bisected it to:


commit ff7bbb9c6ab6e6620429daeff39424bbde1a94b4
Author: Luiz Capitulino <lcapitulino@redhat.com>
Date:   Thu Apr 23 17:12:42 2015 -0400

    kvmclock: set scheduler clock stable

    If you try to enable NOHZ_FULL on a guest today, you'll get
    the following error when the guest tries to deactivate the
    scheduler tick:

     WARNING: CPU: 3 PID: 2182 at kernel/time/tick-sched.c:192 can_stop_full_tick+0xb9/0x290()
     NO_HZ FULL will not work with unstable sched clock
     CPU: 3 PID: 2182 Comm: kworker/3:1 Not tainted 4.0.0-10545-gb9bb6fb #204
     Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
     Workqueue: events flush_to_ldisc
      ffffffff8162a0c7 ffff88011f583e88 ffffffff814e6ba0 0000000000000002
      ffff88011f583ed8 ffff88011f583ec8 ffffffff8104d095 ffff88011f583eb8
      0000000000000000 0000000000000003 0000000000000001 0000000000000001
     Call Trace:
      <IRQ>  [<ffffffff814e6ba0>] dump_stack+0x4f/0x7b
      [<ffffffff8104d095>] warn_slowpath_common+0x85/0xc0
      [<ffffffff8104d146>] warn_slowpath_fmt+0x46/0x50
      [<ffffffff810bd2a9>] can_stop_full_tick+0xb9/0x290
      [<ffffffff810bd9ed>] tick_nohz_irq_exit+0x8d/0xb0
      [<ffffffff810511c5>] irq_exit+0xc5/0x130
      [<ffffffff814f180a>] smp_apic_timer_interrupt+0x4a/0x60
      [<ffffffff814eff5e>] apic_timer_interrupt+0x6e/0x80
      <EOI>  [<ffffffff814ee5d1>] ? _raw_spin_unlock_irqrestore+0x31/0x60
      [<ffffffff8108bbc8>] __wake_up+0x48/0x60
      [<ffffffff8134836c>] n_tty_receive_buf_common+0x49c/0xba0
      [<ffffffff8134a6bf>] ? tty_ldisc_ref+0x1f/0x70
      [<ffffffff81348a84>] n_tty_receive_buf2+0x14/0x20
      [<ffffffff8134b390>] flush_to_ldisc+0xe0/0x120
      [<ffffffff81064d05>] process_one_work+0x1d5/0x540
      [<ffffffff81064c81>] ? process_one_work+0x151/0x540
      [<ffffffff81065191>] worker_thread+0x121/0x470
      [<ffffffff81065070>] ? process_one_work+0x540/0x540
      [<ffffffff8106b4df>] kthread+0xef/0x110
      [<ffffffff8106b3f0>] ? __kthread_parkme+0xa0/0xa0
      [<ffffffff814ef4f2>] ret_from_fork+0x42/0x70
      [<ffffffff8106b3f0>] ? __kthread_parkme+0xa0/0xa0
     ---[ end trace 06e3507544a38866 ]---

    However, it turns out that kvmclock does provide a stable
    sched_clock callback. So, let the scheduler know this which
    in turn makes NOHZ_FULL work in the guest.

    Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
    Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>


Thanks,
Sasha