linux-kernel - Re: [PATCH] thermal/powerclamp: Prevent division by zero when counting interval

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Message-ID: <20160804103200.486fb236@jacob-builder>
Date:	Thu, 4 Aug 2016 10:32:00 -0700
From:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Zhang Rui <rui.zhang@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH] thermal/powerclamp: Prevent division by zero when
 counting interval

On Thu,  4 Aug 2016 16:56:46 +0200
Petr Mladek <pmladek@...e.com> wrote:

> I have got a zero division error when disabling the forced
> idle injection from the intel powerclamp. I did
> 
>   echo 0 >/sys/class/thermal/cooling_device48/cur_state
> 
> and got
> 
> [  986.072632] divide error: 0000 [#1] PREEMPT SMP
> [  986.078989] Modules linked in:
> [  986.083618] CPU: 17 PID: 24967 Comm: kidle_inject/17 Not tainted
> 4.7.0-1-default+ #3055 [  986.093781] Hardware name: Intel
> Corporation S2600CP/S2600CP, BIOS RMLSDP.86I.R3.27.D685.1305151734
> 05/15/2013 [  986.106227] task: ffff880430e1c080 task.stack:
> ffff880427ef0000 [  986.114122] RIP: 0010:[<ffffffff81794859>]
> [<ffffffff81794859>] clamp_thread+0x1d9/0x600 [  986.124609] RSP:
> 0018:ffff880427ef3e20  EFLAGS: 00010246 [  986.131860] RAX:
> 0000000000000258 RBX: 0000000000000006 RCX: 0000000000000001
> [  986.141179] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
> 0000000000000018 [  986.150478] RBP: ffff880427ef3ec8 R08:
> ffff880427ef0000 R09: 0000000000000002 [  986.159779] R10:
> 0000000000003df2 R11: 0000000000000018 R12: 0000000000000002
> [  986.169089] R13: 0000000000000000 R14: ffff880427ef0000 R15:
> ffff880427ef0000 [  986.178388] FS:  0000000000000000(0000)
> GS:ffff880435940000(0000) knlGS:0000000000000000 [  986.188785] CS:
> 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [  986.196559] CR2:
> 00007f1d0caf0000 CR3: 0000000002006000 CR4: 00000000001406e0
> [  986.205909] Stack: [  986.209524]  ffff8802be897b00
> ffff880430e1c080 0000000000000011 0000006a35959780 [  986.219236]
> 0000000000000011 ffff880427ef0008 0000000000000000 ffff8804359503d0
> [  986.228966]  0000000100029d93 ffffffff81794140 0000000000000000
> ffffffff05000011 [  986.238686] Call Trace: [  986.242825]
> [<ffffffff81794140>] ? pkg_state_counter+0x80/0x80 [  986.250866]
> [<ffffffff81794680>] ? powerclamp_set_cur_state+0x180/0x180
> [  986.259797]  [<ffffffff8111d1a9>] kthread+0xc9/0xe0
> [  986.266682]  [<ffffffff8193d69f>] ret_from_fork+0x1f/0x40
> [  986.274142]  [<ffffffff8111d0e0>] ?
> kthread_create_on_node+0x180/0x180 [  986.282869] Code: d1 ea 48 89
> d6 80 3d 6a d0 d4 00 00 ba 64 00 00 00 89 d8 41 0f 45 f5 0f af c2 42
> 8d 14 2e be 31 00 00 00 83 fa 31 0f 42 f2 31 d2 <f7> f6 48 8b 15 9e
> 07 87 00 48 8b 3d 97 07 87 00 48 63 f0 83 e8 [  986.307806] RIP
> [<ffffffff81794859>] clamp_thread+0x1d9/0x600 [  986.315871]  RSP
> <ffff880427ef3e20>
> 
> RIP points to the following lines:
> 
> 	compensation = get_compensation(target_ratio);
> 	interval = duration_jiffies*100/(target_ratio+compensation);
> 
> A solution would be to switch the following two commands in
> powerclamp_set_cur_state():
> 
> 	set_target_ratio = 0;
> 	end_power_clamp();
> 
I see, there is race condition, clamping threads should be stopped if
target ratio is 0.
> But I think that the zero division might happen also when target_ratio
> is non-zero because the compensation might be negative. Therefore
> it is better to check the sum of target_ratio and compensation
> explicitly.
> 
compensation should never be negative. since it is the additional idle
ratio added on top of requested ratio. If actual idle is more than
requested, we will skip injection period.
So i prefer to have both changes.
> Also the compensated_ratio variable is always set. Therefore there
> is no need to initialize it.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  drivers/thermal/intel_powerclamp.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/thermal/intel_powerclamp.c
> b/drivers/thermal/intel_powerclamp.c index 015ce2eb6eb7..65b865184990
> 100644 --- a/drivers/thermal/intel_powerclamp.c
> +++ b/drivers/thermal/intel_powerclamp.c
> @@ -388,7 +388,7 @@ static int clamp_thread(void *arg)
>  		int sleeptime;
>  		unsigned long target_jiffies;
>  		unsigned int guard;
> -		unsigned int compensation = 0;
> +		unsigned int compensated_ratio;
>  		int interval; /* jiffies to sleep for each attempt */
>  		unsigned int duration_jiffies =
> msecs_to_jiffies(duration); unsigned int window_size_now;
> @@ -409,8 +409,11 @@ static int clamp_thread(void *arg)
>  		 * c-states, thus we need to compensate the injected
> idle ratio
>  		 * to achieve the actual target reported by the HW.
>  		 */
> -		compensation = get_compensation(target_ratio);
> -		interval =
> duration_jiffies*100/(target_ratio+compensation);
> +		compensated_ratio = target_ratio +
> +			get_compensation(target_ratio);
> +		if (compensated_ratio <= 0)
> +			compensated_ratio = 1;
> +		interval = duration_jiffies * 100 /
> compensated_ratio; 
>  		/* align idle time */
>  		target_jiffies = roundup(jiffies, interval);

[Jacob Pan]