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From: Oleksandr Natalenko <oleksandr@natalenko.name>
To: Jean Delvare <jdelvare@suse.de>
Cc: Jonathan Buzzard <jonathan@buzzard.org.uk>,
        Pali =?ISO-8859-1?Q?Roh=E1r?= <pali.rohar@gmail.com>,
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        Mario Limonciello <mario.limonciello@dell.com>
Subject: Re: i8k_smm_func() takes enormous of time to execute
Date: Fri, 24 Nov 2017 12:17:30 +0100
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Hi.

On pátek 24. listopadu 2017 11:22:28 CET Jean Delvare wrote:
> Hi Oleksandr,
> 
> On jeu., 2017-11-23 at 22:41 +0100, Oleksandr Natalenko wrote:
> > I've noticed that querying Dell Vostro 3360 hwmon sensor freezes the
> > system
> > completely (like, really completely, even mouse cursor does not move, and
> > sound playback stops) for approx. half of a second. Then, system recovers
> > and runs as usual. Also, sensor readings are okay, for instance:
> > 
> > ===
> > $ sensors dell_smm-virtual-0
> > dell_smm-virtual-0
> > Adapter: Virtual device
> > Processor Fan: 5615 RPM
> > CPU:            +51.0°C
> > Ambient:        +36.0°C
> > Other:          +63.0°C
> > SODIMM:         +41.0°C
> > ===
> > 
> > So, I've used trace-cmd to check what takes that much amount of time to
> > execute, and got this:
> > 
> > ===
> > # trace-cmd record -p function_graph -l i8k_smm -F sensors
> > dell_smm-virtual-0 $ trace-cmd report
> > ...
> > sensors-23694 [002] 89099.214369: funcgraph_entry:      # 503440.746 us |
> > i8k_smm();
> > ...
> > ===
> > 
> > Clearly, 0.5 s delay.
> > 
> > Looking at i8k_smm(), it calls i8k_smm_func() on 0th CPU:
> > 
> > ===
> > 
> >  232     ret = smp_call_on_cpu(0, i8k_smm_func, regs, true);
> > 
> > ===
> > 
> > which, in turn, does some asm magic.
> > 
> > I know that SMM is kinda "black box", and kernel has little to do with it,
> > but I think that under Windows, for instance, it would work without
> > freezes. So, likely, querying SMM might be done differently.
> > 
> > I do not know how to approach this issue, thus asking for help/advice.
> > Also, CCing Jonathan since the comment before asm magic says this:
> > 
> > ===
> > 
> >  137  * Call the System Management Mode BIOS. Code provided by Jonathan
> > 
> > Buzzard.
> > ===
> > 
> > This was also reported in various places before, for instance, [1], but
> > unfortunately, without any solution.
> 
> There are two patches waiting to be tested in
> https://bugzilla.kernel.org/show_bug.cgi?id=195751

Tested and attached a couple of patches on top of those to the BZ. If 
disabling fan control is the only approach here, I can confirm that this 
works.

Thanks.

Regards,
  Oleksandr