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

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Date:	Fri, 5 Aug 2016 15:12:42 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Zhang Rui <rui.zhang@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thermal/powerclamp: Prevent division by zero when
 counting interval

On Thu 2016-08-04 10:32:00, Jacob Pan wrote:
> 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.

I am not sure if you are talking about the desired behavior or the
current code. get_compensation() returns value computed from
steady_comp values. These values are assigned in adjust_compensation()
and the code seems to store even negative values. But I did not
tried to investigate it much deeper.

> If actual idle is more than requested, we will skip injection period.
> So i prefer to have both changes.

OK, I'll send an updated patch.

Best Regards,
Petr