Re: [PATCH] cpufreq, Fix overflow in busy_scaled due to long delay [v2]

[Prarit Bhargava]

On 06/15/2015 06:12 PM, Kristen Carlson Accardi wrote:
> On Mon, 15 Jun 2015 13:43:29 -0400
> Prarit Bhargava <prarit@xxxxxxxxxx> 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.

P.

> 
> Thanks,
> Kristen
> 
>>
>> Cc: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
>> Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
>> Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>> Cc: Doug Smythies <dsmythies@xxxxxxxxx>
>> Cc: linux-pm@xxxxxxxxxxxxxxx
>>
>> Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx>
>> ---
>>  drivers/cpufreq/intel_pstate.c |   10 +++++-----
>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
>> index 6414661..b153d86 100644
>> --- a/drivers/cpufreq/intel_pstate.c
>> +++ b/drivers/cpufreq/intel_pstate.c
>> @@ -48,9 +48,9 @@ static inline int32_t mul_fp(int32_t x, int32_t y)
>>  	return ((int64_t)x * (int64_t)y) >> FRAC_BITS;
>>  }
>>  
>> -static inline int32_t div_fp(int32_t x, int32_t y)
>> +static inline int32_t div_fp(s64 x, s64 y)
>>  {
>> -	return div_s64((int64_t)x << FRAC_BITS, y);
>> +	return div64_s64((int64_t)x << FRAC_BITS, y);
>>  }
>>  
>>  static inline int ceiling_fp(int32_t x)
>> @@ -794,7 +794,7 @@ static inline void intel_pstate_set_sample_time(struct cpudata *cpu)
>>  static inline int32_t intel_pstate_get_scaled_busy(struct cpudata *cpu)
>>  {
>>  	int32_t core_busy, max_pstate, current_pstate, sample_ratio;
>> -	u32 duration_us;
>> +	s64 duration_us;
>>  	u32 sample_time;
>>  
>>  	/*
>> @@ -821,8 +821,8 @@ static inline int32_t intel_pstate_get_scaled_busy(struct cpudata *cpu)
>>  	 * to adjust our busyness.
>>  	 */
>>  	sample_time = pid_params.sample_rate_ms  * USEC_PER_MSEC;
>> -	duration_us = (u32) ktime_us_delta(cpu->sample.time,
>> -					   cpu->last_sample_time);
>> +	duration_us = ktime_us_delta(cpu->sample.time,
>> +				     cpu->last_sample_time);
>>  	if (duration_us > sample_time * 3) {
>>  		sample_ratio = div_fp(int_tofp(sample_time),
>>  				      int_tofp(duration_us));
> 
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