From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sasha Levin Subject: kvm: odd time values since "kvmclock: set scheduler clock stable" Date: Tue, 12 May 2015 19:17:24 -0400 Message-ID: <55528A04.2060404@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: mtosatti@redhat.com, Paolo Bonzini , lcapitulino@redhat.com To: KVM General , LKML Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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 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: [] dump_stack+0x4f/0x7b [] warn_slowpath_common+0x85/0xc0 [] warn_slowpath_fmt+0x46/0x50 [] can_stop_full_tick+0xb9/0x290 [] tick_nohz_irq_exit+0x8d/0xb0 [] irq_exit+0xc5/0x130 [] smp_apic_timer_interrupt+0x4a/0x60 [] apic_timer_interrupt+0x6e/0x80 [] ? _raw_spin_unlock_irqrestore+0x31/0x60 [] __wake_up+0x48/0x60 [] n_tty_receive_buf_common+0x49c/0xba0 [] ? tty_ldisc_ref+0x1f/0x70 [] n_tty_receive_buf2+0x14/0x20 [] flush_to_ldisc+0xe0/0x120 [] process_one_work+0x1d5/0x540 [] ? process_one_work+0x151/0x540 [] worker_thread+0x121/0x470 [] ? process_one_work+0x540/0x540 [] kthread+0xef/0x110 [] ? __kthread_parkme+0xa0/0xa0 [] ret_from_fork+0x42/0x70 [] ? __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 Signed-off-by: Luiz Capitulino Signed-off-by: Paolo Bonzini Thanks, Sasha