Messages in this thread | | | From | Rui Salvaterra <> | Date | Tue, 28 Apr 2020 08:27:06 +0100 | Subject | Re: [BISECTED] bug/regression, x86-64: completely unbootable machine |
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On Sat, 25 Apr 2020 at 17:01, Giovanni Gherdovich <ggherdovich@suse.cz> wrote: > > 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
Hi again, Giovanni,
Thanks for the detailed and insightful explanation and sorry for not replying earlier. This was indeed the bug I was hitting, as my Arrandale laptop is now booting 5.7-rc3 just fine.
Best regards, Rui
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