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Message-ID: <1587830499.28179.66.camel@suse.cz>
Date: Sat, 25 Apr 2020 18:01:39 +0200
From: Giovanni Gherdovich <ggherdovich@...e.cz>
To: Rui Salvaterra <rsalvaterra@...il.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [BISECTED] bug/regression, x86-64: completely unbootable machine
On Fri, 2020-04-24 at 18:39 +0100, Rui Salvaterra wrote:
> Hi, again,
>
> On Fri, 24 Apr 2020 at 09:11, Giovanni Gherdovich <ggherdovich@...e.cz> wrote:
> >
> > The problem you encountered is due to a bug where the code doesn't work on
> > machines with less than 4 physical CPU cores.
>
> Thinking about it more thoroughly, are you sure that's the bug I'm
> hitting? I'm asking because I have an i5-6200U (Skylake, also dual
> core, dual thread, like the i3-380M) laptop which runs 5.7-rc1/rc2
> just fine.
>
> Thanks,
> Rui
There is an easy way to tell (besides compiling with those patches on top and
check if it works): run the command "turbostat --interval 1 sleep 0", the
output should tell you the content of the register MSR_TURBO_RATIO_LIMIT.
If bits 31:24 are zero, you see the bug (the code divides by that value),
otherwise you don't. Some 2 cores / 4 threads CPU have a non-zero value there
(even if it doesn't mean much), some others have zero instead.
The Intel Software Developer Manual (SDM) says the register content is like
this:
Bit Fields Bit Description
7:0 Maximum turbo ratio limit of 1 core active.
15:8 Maximum turbo ratio limit of 2 core active.
23:16 Maximum turbo ratio limit of 3 core active.
31:24 Maximum turbo ratio limit of 4 core active.
39:32 Maximum turbo ratio limit of 5 core active.
47:40 Maximum turbo ratio limit of 6 core active.
55:48 Maximum turbo ratio limit of 7 core active.
63:56 Maximum turbo ratio limit of 8 core active.
As I wrote above, some 2c/4t CPUs will say (correctly) their 4 cores turbo
frequency is zero, such as this Intel Core i5-430M (Arrandale) where I've seen
turbostat saying:
cpu1: MSR_TURBO_RATIO_LIMIT: 0x00001313
19 * 133.3 = 2533.3 MHz max turbo 2 active cores
19 * 133.3 = 2533.3 MHz max turbo 1 active cores
On the contrary, my laptop has an Intel Core i5-5300U (Broadwell, also
2 cores / 4 threads) and it has:
cpu3: MSR_TURBO_RATIO_LIMIT: 0x1b1b1b1b1b1d
27 * 100.0 = 2700.0 MHz max turbo 6 active cores
27 * 100.0 = 2700.0 MHz max turbo 5 active cores
27 * 100.0 = 2700.0 MHz max turbo 4 active cores
27 * 100.0 = 2700.0 MHz max turbo 3 active cores
27 * 100.0 = 2700.0 MHz max turbo 2 active cores
29 * 100.0 = 2900.0 MHz max turbo 1 active cores
You can see above that the 4 cores turbo freq is declared as 2.7GHz even if
it's nonsense because there aren't 4 cores. In any case, this cpu wouldn't
trigger the bug, just as your skylake.
Thanks,
Giovanni
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