It starts with a WARNING in intel_crtc_wait_for_pending_flips() and then after a few very slow (tens of seconds) VT switches, in intel_display_power_put()

Sep 18 06:20:00 tito kernel: [  801.637835] ------------[ cut here ]------------
Sep 18 06:20:00 tito kernel: [  801.637876] WARNING: CPU: 2 PID: 1386 at /build/buildd/linux-3.16.0/drivers/gpu/drm/i915/intel_display.c:3313 intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]()
Sep 18 06:20:00 tito kernel: [  801.637878] Modules linked in: xt_recent ctr ccm pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core v4l2
_common videodev btusb media hid_multitouch nvram dm_crypt arc4 ip6t_REJECT xt_hl ip6t_rt nf_conntrack_ipv6 nf_defrag_ipv6 ipt_REJECT xt_comment xt_LOG xt_limit xt_tcpudp joydev xt_addrtype nf_conntrack_ipv4
 nf_defrag_ipv4 xt_conntrack intel_rapl x86_pkg_temp_thermal intel_powerclamp ip6table_filter ip6_tables coretemp nf_conntrack_netbios_ns nf_conntrack_broadcast nf_nat_ftp nf_nat nf_conntrack_ftp nf_conntrac
k kvm_intel hid_rmi dell_wmi iptable_filter sparse_keymap kvm ip_tables x_tables iwlmvm mac80211 crct10dif_pclmul dell_laptop dcdbas snd_hda_codec_realtek snd_hda_codec_generic crc32_pclmul snd_hda_codec_hdm
i ghash_clmulni_intel aesni_intel iwlwifi snd_hda_intel aes_x86_64 snd_hda_controller lrw snd_hda_codec gf128mul glue_helper ablk_helper snd_soc_rt5640 cryptd snd_hwdep snd_soc_rl6231 snd_soc_core cfg80211 s
erio_raw snd_compress snd_pcm_dmaengine snd_pcm mei_me shpchp snd_seq_midi mei lpc_ich snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer snd snd_soc_sst_acpi dw_dmac soundcore i2c_hid dw_dmac_c
ore i2c_designware_platform i2c_designware_core 8250_dw spi_pxa2xx_platform rfcomm bnep bluetooth 6lowpan_iphc mac_hid parport_pc ppdev lp parport hid_generic usbhid hid i915 i2c_algo_bit drm_kms_helper drm 
psmouse ahci libahci wmi video sdhci_acpi sdhci
Sep 18 06:20:00 tito kernel: [  801.637940] CPU: 2 PID: 1386 Comm: Xorg Tainted: G           OE 3.16.0-14-generic #20-Ubuntu
Sep 18 06:20:00 tito kernel: [  801.637941] Hardware name: Dell Inc. XPS13 9333/0D13CR, BIOS A04 03/19/2014
Sep 18 06:20:00 tito kernel: [  801.637943]  0000000000000009 ffff880036543c18 ffffffff817793ae 0000000000000000
Sep 18 06:20:00 tito kernel: [  801.637945]  ffff880036543c50 ffffffff8106fced 0000000000000000 ffff88020cbfb000
Sep 18 06:20:00 tito kernel: [  801.637947]  ffff88020c488240 ffff88020c8bd800 ffff88020c8bd800 ffff880036543c60
Sep 18 06:20:00 tito kernel: [  801.637950] Call Trace:
Sep 18 06:20:00 tito kernel: [  801.637957]  [<ffffffff817793ae>] dump_stack+0x45/0x56
Sep 18 06:20:00 tito kernel: [  801.637961]  [<ffffffff8106fced>] warn_slowpath_common+0x7d/0xa0
Sep 18 06:20:00 tito kernel: [  801.637964]  [<ffffffff8106fdca>] warn_slowpath_null+0x1a/0x20
Sep 18 06:20:00 tito kernel: [  801.637983]  [<ffffffffc0268e3c>] intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]
Sep 18 06:20:00 tito kernel: [  801.637987]  [<ffffffff810b94d0>] ? prepare_to_wait_event+0x100/0x100
Sep 18 06:20:00 tito kernel: [  801.638005]  [<ffffffffc026b903>] intel_crtc_disable_planes+0x33/0x1c0 [i915]
Sep 18 06:20:00 tito kernel: [  801.638007]  [<ffffffff8177e58b>] ? __ww_mutex_lock+0x1b/0xb0
Sep 18 06:20:00 tito kernel: [  801.638024]  [<ffffffffc026c857>] haswell_crtc_disable+0x57/0x2e0 [i915]
Sep 18 06:20:00 tito kernel: [  801.638026]  [<ffffffff8177f1e2>] ? mutex_lock+0x12/0x30
Sep 18 06:20:00 tito kernel: [  801.638043]  [<ffffffffc026d297>] intel_crtc_update_dpms+0x67/0xa0 [i915]
Sep 18 06:20:00 tito kernel: [  801.638061]  [<ffffffffc0271909>] intel_connector_dpms+0x59/0x70 [i915]
Sep 18 06:20:00 tito kernel: [  801.638079]  [<ffffffffc01afdf9>] drm_mode_obj_set_property_ioctl+0x399/0x3a0 [drm]
Sep 18 06:20:00 tito kernel: [  801.638092]  [<ffffffffc01afe30>] drm_mode_connector_property_set_ioctl+0x30/0x40 [drm]
Sep 18 06:20:00 tito kernel: [  801.638102]  [<ffffffffc019ea3f>] drm_ioctl+0x1df/0x680 [drm]
Sep 18 06:20:00 tito kernel: [  801.638108]  [<ffffffff810a9d18>] ? __enqueue_entity+0x78/0x80
Sep 18 06:20:00 tito kernel: [  801.638111]  [<ffffffff8101e0ef>] ? __switch_to_xtra+0x13f/0x170
Sep 18 06:20:00 tito kernel: [  801.638114]  [<ffffffff811f4058>] do_vfs_ioctl+0x2c8/0x4a0
Sep 18 06:20:00 tito kernel: [  801.638117]  [<ffffffff8177c07d>] ? __schedule+0x39d/0x890
Sep 18 06:20:00 tito kernel: [  801.638119]  [<ffffffff811f42b1>] SyS_ioctl+0x81/0xa0
Sep 18 06:20:00 tito kernel: [  801.638122]  [<ffffffff8178142d>] system_call_fastpath+0x1a/0x1f
Sep 18 06:20:00 tito kernel: [  801.638124] ---[ end trace ec27fd2563ff929d ]---


Sep 18 05:59:57 tito kernel: [46236.672315] ------------[ cut here ]------------
Sep 18 05:59:57 tito kernel: [46236.672374] WARNING: CPU: 0 PID: 1437 at /build/buildd/linux-3.16.0/drivers/gpu/drm/i915/intel_pm.c:5997 intel_display_power_put+0x14c/0x160 [i915]()
Sep 18 05:59:57 tito kernel: [46236.672377] Modules linked in: ctr ccm pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core v4l2_common videodev media btusb hid_multitouch nvram dm_crypt joydev ip6t_REJECT xt_hl ip6t_rt nf_conntrack_ipv6 nf_defrag_ipv6 ipt_REJECT xt_comment xt_LOG xt_limit xt_tcpudp xt_addrtype arc4 intel_rapl nf_conntrack_ipv4 nf_defrag_ipv4 x86_pkg_temp_thermal xt_conntrack intel_powerclamp coretemp ip6table_filter ip6_tables kvm_intel nf_conntrack_netbios_ns hid_rmi nf_conntrack_broadcast nf_nat_ftp nf_nat dell_wmi nf_conntrack_ftp sparse_keymap nf_conntrack kvm iptable_filter ip_tables x_tables crct10dif_pclmul crc32_pclmul dell_laptop dcdbas ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd iwlmvm mac80211 snd_hda_codec_realtek serio_raw snd_hda_codec_generic iwlwifi cfg80211 rfcomm snd_hda_codec_hdmi bnep bluetooth lpc_ich 6lowpan_iphc shpchp mei_me snd_hda_intel snd_hda_controller snd_hda_codec snd_hwdep mei snd_soc_rt5640 snd_soc_rl6231 snd_soc_core snd_compress snd_pcm_dmaengine snd_pcm snd_seq_midi snd_seq_midi_event dw_dmac dw_dmac_core 8250_dw snd_rawmidi snd_soc_sst_acpi snd_seq snd_seq_device snd_timer snd i2c_hid soundcore i2c_designware_platform spi_pxa2xx_platform i2c_designware_core mac_hid parport_pc ppdev lp parport hid_generic usbhid hid i915 i2c_algo_bit drm_kms_helper psmouse drm ahci wmi libahci video sdhci_acpi sdhci
Sep 18 05:59:57 tito kernel: [46236.672493] CPU: 0 PID: 1437 Comm: Xorg Tainted: G        W  OE 3.16.0-14-generic #20-Ubuntu
Sep 18 05:59:57 tito kernel: [46236.672496] Hardware name: Dell Inc. XPS13 9333/0D13CR, BIOS A04 03/19/2014
Sep 18 05:59:57 tito kernel: [46236.672499]  0000000000000009 ffff8802101dfaf8 ffffffff817793ae 0000000000000000
Sep 18 05:59:57 tito kernel: [46236.672504]  ffff8802101dfb30 ffffffff8106fced ffff88020cab002c 000000000000000b
Sep 18 05:59:57 tito kernel: [46236.672509]  ffff88020cab8550 ffff88020fda5000 ffff88020cab0000 ffff8802101dfb40
Sep 18 05:59:57 tito kernel: [46236.672513] Call Trace:
Sep 18 05:59:57 tito kernel: [46236.672526]  [<ffffffff817793ae>] dump_stack+0x45/0x56
Sep 18 05:59:57 tito kernel: [46236.672535]  [<ffffffff8106fced>] warn_slowpath_common+0x7d/0xa0
Sep 18 05:59:57 tito kernel: [46236.672540]  [<ffffffff8106fdca>] warn_slowpath_null+0x1a/0x20
Sep 18 05:59:57 tito kernel: [46236.672567]  [<ffffffffc02bf25c>] intel_display_power_put+0x14c/0x160 [i915]
Sep 18 05:59:57 tito kernel: [46236.672606]  [<ffffffffc0307e34>] modeset_update_crtc_power_domains+0x204/0x210 [i915]
Sep 18 05:59:57 tito kernel: [46236.672643]  [<ffffffffc0307e4e>] haswell_modeset_global_resources+0xe/0x10 [i915]
Sep 18 05:59:57 tito kernel: [46236.672678]  [<ffffffffc030954a>] __intel_set_mode+0x5fa/0xab0 [i915]
Sep 18 05:59:57 tito kernel: [46236.672714]  [<ffffffffc030c936>] intel_set_mode+0x16/0x30 [i915]
Sep 18 05:59:57 tito kernel: [46236.672749]  [<ffffffffc030d9ba>] intel_crtc_set_config+0xa4a/0xda0 [i915]
Sep 18 05:59:57 tito kernel: [46236.672780]  [<ffffffffc02296c1>] drm_mode_set_config_internal+0x61/0xe0 [drm]
Sep 18 05:59:57 tito kernel: [46236.672808]  [<ffffffffc022d403>] drm_mode_setcrtc+0x283/0x580 [drm]
Sep 18 05:59:57 tito kernel: [46236.672827]  [<ffffffffc021da3f>] drm_ioctl+0x1df/0x680 [drm]
Sep 18 05:59:57 tito kernel: [46236.672840]  [<ffffffff811f4058>] do_vfs_ioctl+0x2c8/0x4a0
Sep 18 05:59:57 tito kernel: [46236.672845]  [<ffffffff811e26a1>] ? __sb_end_write+0x31/0x60
Sep 18 05:59:57 tito kernel: [46236.672851]  [<ffffffff811e0212>] ? vfs_write+0x1b2/0x1f0
Sep 18 05:59:57 tito kernel: [46236.672856]  [<ffffffff811f42b1>] SyS_ioctl+0x81/0xa0
Sep 18 05:59:57 tito kernel: [46236.672861]  [<ffffffff8178142d>] system_call_fastpath+0x1a/0x1f
Sep 18 05:59:57 tito kernel: [46236.672864] ---[ end trace 9b468c36efd27191 ]---
Sep 18 05:59:57 tito kernel: [46236.672867] ------------[ cut here ]------------