Date:	Tue, 16 Jun 2015 23:35:48 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	Kristen Carlson Accardi <kristen@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Doug Smythies <dsmythies@...us.net>, linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpufreq, Fix overflow in busy_scaled due to long delay [v2]

On Tuesday, June 16, 2015 11:35:10 PM Rafael J. Wysocki wrote:
> On Monday, June 15, 2015 07:33:03 PM Prarit Bhargava wrote:
> > 
> > On 06/15/2015 06:12 PM, Kristen Carlson Accardi wrote:
> > > On Mon, 15 Jun 2015 13:43:29 -0400
> > > Prarit Bhargava <prarit@...hat.com> wrote:
> > > 
> > >> The kernel may delay interrupts for a long time which can result in timers
> > >> being delayed. If this occurs the intel_pstate driver will crash with a
> > >> divide by zero error:
> > >>
> > >> divide error: 0000 [#1] SMP
> > >> Modules linked in: btrfs zlib_deflate raid6_pq xor msdos ext4 mbcache jbd2 binfmt_misc arc4 md4 nls_utf8 cifs dns_resolver tcp_lp bnep bluetooth rfkill fuse dm_service_time iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi nf_conntrack_netbios_ns nf_conntrack_broadcast nf_conntrack_ftp ip6t_rpfilter ip6t_REJECT ipt_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw iptable_filter ip_tables intel_powerclamp coretemp vfat fat kvm_intel iTCO_wdt iTCO_vendor_support ipmi_devintf sr_mod kvm crct10dif_pclmul
> > >>  crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel cdc_ether lrw usbnet cdrom mii gf128mul glue_helper ablk_helper cryptd lpc_ich mfd_core pcspkr sb_edac edac_core ipmi_si ipmi_msghandler ioatdma wmi shpchp acpi_pad nfsd auth_rpcgss nfs_acl lockd uinput dm_multipath sunrpc xfs libcrc32c usb_storage sd_mod crc_t10dif crct10dif_common ixgbe mgag200 syscopyarea sysfillrect sysimgblt mdio drm_kms_helper ttm igb drm ptp pps_core dca i2c_algo_bit megaraid_sas i2c_core dm_mirror dm_region_hash dm_log dm_mod
> > >> CPU: 113 PID: 0 Comm: swapper/113 Tainted: G        W   --------------   3.10.0-229.1.2.el7.x86_64 #1
> > >> Hardware name: IBM x3950 X6 -[3837AC2]-/00FN827, BIOS -[A8E112BUS-1.00]- 08/27/2014
> > >> task: ffff880fe8abe660 ti: ffff880fe8ae4000 task.ti: ffff880fe8ae4000
> > >> RIP: 0010:[<ffffffff814a9279>]  [<ffffffff814a9279>] intel_pstate_timer_func+0x179/0x3d0
> > >> RSP: 0018:ffff883fff4e3db8  EFLAGS: 00010206
> > >> RAX: 0000000027100000 RBX: ffff883fe6965100 RCX: 0000000000000000
> > >> RDX: 0000000000000000 RSI: 0000000000000010 RDI: 000000002e53632d
> > >> RBP: ffff883fff4e3e20 R08: 000e6f69a5a125c0 R09: ffff883fe84ec001
> > >> R10: 0000000000000002 R11: 0000000000000005 R12: 00000000000049f5
> > >> R13: 0000000000271000 R14: 00000000000049f5 R15: 0000000000000246
> > >> FS:  0000000000000000(0000) GS:ffff883fff4e0000(0000) knlGS:0000000000000000
> > >> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >> CR2: 00007f7668601000 CR3: 000000000190a000 CR4: 00000000001407e0
> > >> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > >> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> > >> Stack:
> > >>  ffff883fff4e3e58 ffffffff81099dc1 0000000000000086 0000000000000071
> > >>  ffff883fff4f3680 0000000000000071 fbdc8a965e33afee ffffffff810b69dd
> > >>  ffff883fe84ec000 ffff883fe6965108 0000000000000100 ffffffff814a9100
> > >> Call Trace:
> > >>  <IRQ>
> > >>
> > >>  [<ffffffff81099dc1>] ? run_posix_cpu_timers+0x51/0x840
> > >>  [<ffffffff810b69dd>] ? trigger_load_balance+0x5d/0x200
> > >>  [<ffffffff814a9100>] ? pid_param_set+0x130/0x130
> > >>  [<ffffffff8107df56>] call_timer_fn+0x36/0x110
> > >>  [<ffffffff814a9100>] ? pid_param_set+0x130/0x130
> > >>  [<ffffffff8107fdcf>] run_timer_softirq+0x21f/0x320
> > >>  [<ffffffff81077b2f>] __do_softirq+0xef/0x280
> > >>  [<ffffffff816156dc>] call_softirq+0x1c/0x30
> > >>  [<ffffffff81015d95>] do_softirq+0x65/0xa0
> > >>  [<ffffffff81077ec5>] irq_exit+0x115/0x120
> > >>  [<ffffffff81616355>] smp_apic_timer_interrupt+0x45/0x60
> > >>  [<ffffffff81614a1d>] apic_timer_interrupt+0x6d/0x80
> > >>  <EOI>
> > >>
> > >>  [<ffffffff814a9c32>] ? cpuidle_enter_state+0x52/0xc0
> > >>  [<ffffffff814a9c28>] ? cpuidle_enter_state+0x48/0xc0
> > >>  [<ffffffff814a9d65>] cpuidle_idle_call+0xc5/0x200
> > >>  [<ffffffff8101d14e>] arch_cpu_idle+0xe/0x30
> > >>  [<ffffffff810c67c1>] cpu_startup_entry+0xf1/0x290
> > >>  [<ffffffff8104228a>] start_secondary+0x1ba/0x230
> > >> Code: 42 0f 00 45 89 e6 48 01 c2 43 8d 44 6d 00 39 d0 73 26 49 c1 e5 08 89 d2 4d 63 f4 49 63 c5 48 c1 e2 08 48 c1 e0 08 48 63 ca 48 99 <48> f7 f9 48 98 4c 0f af f0 49 c1 ee 08 8b 43 78 c1 e0 08 44 29
> > >> RIP  [<ffffffff814a9279>] intel_pstate_timer_func+0x179/0x3d0
> > >>  RSP <ffff883fff4e3db8>
> > >>
> > >> The kernel values for cpudata for CPU 113 were:
> > >>
> > >> struct cpudata {
> > >>   cpu = 113,
> > >>   timer = {
> > >>     entry = {
> > >>       next = 0x0,
> > >>       prev = 0xdead000000200200
> > >>     },
> > >>     expires = 8357799745,
> > >>     base = 0xffff883fe84ec001,
> > >>     function = 0xffffffff814a9100 <intel_pstate_timer_func>,
> > >>     data = 18446612406765768960,
> > >> <snip>
> > >>     i_gain = 0,
> > >>     d_gain = 0,
> > >>     deadband = 0,
> > >>     last_err = 22489
> > >>   },
> > >>   last_sample_time = {
> > >>     tv64 = 4063132438017305
> > >>   },
> > >>   prev_aperf = 287326796397463,
> > >>   prev_mperf = 251427432090198,
> > >>   sample = {
> > >>     core_pct_busy = 23081,
> > >>     aperf = 2937407,
> > >>     mperf = 3257884,
> > >>     freq = 2524484,
> > >>     time = {
> > >>       tv64 = 4063149215234118
> > >>     }
> > >>   }
> > >> }
> > >>
> > >> which results in the time between samples = last_sample_time - sample.time
> > >> = 4063149215234118 - 4063132438017305 = 16777216813 which is 16.777 seconds.
> > >>
> > >> The duration between reads of the APERF and MPERF registers overflowed a s32
> > >> sized integer in intel_pstate_get_scaled_busy()'s call to div_fp().  The result
> > >> is that int_tofp(duration_us) == 0, and the kernel attempts to divide by 0.
> > >>
> > >> While the kernel shouldn't be delaying for a long time, it can and does
> > >> happen and the intel_pstate driver should not panic in this situation.  This
> > >> patch changes the div_fp() function to use div64_s64() to allow for "long"
> > >> division.  This will avoid the overflow condition on long delays.
> > >>
> > >> [v2]: use div64_s64() in div_fp()
> > > 
> > > Were you able to resolve your original concerns with doing this?  I
> > > thought you mentioned that you'd tested it and it gave you some
> > > negative side effects?
> > 
> > Sorry Kristen, I think my comment confused you.  I was worried about passing "0"
> > back as the core_busy value after a long delay.  Given that it is already
> > possible to return 0 when the duration exceeds 3 * the expected delay I no
> > longer think there is any issue of returning 0 on a _really really really long_
> > delay.
> > 
> > IOW, there isn't a problem and I was being overly careful with v1 of the patch.
> 
> OK, patch queued up fo 4.1, thanks!

That should have been 4.2, sorry.


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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