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07-09-2020

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Ideapad 15ARE05 - TSC broken

2020-07-09, 13:55 PM

Issue is identical to this thread: https://forums.lenovo.com/t5/Linux-Discussion/ThinkPad-A485-TSC-broken-on-BIOS-1-16/m-p/4354663 , same dmesg errors and the system falls back to hpet clock source which is making my application very slow. The CPU I have is the 4800U.

 

I'm using BIOS  E7CN24WW , latest one is E7CN25WW on https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/5-series/ideapad-5-15are05/downloads/DS544050 but the changelog doesn't mention TSC and I can't update the BIOS as I only have Linux.

 

(Offtopic - is it possible to flash the BIOS rather than running an executable on windows? Alternatively, does it work if windows is on a USB drive?)

 

I know the machine isn't validated for Linux and there are higher end laptops for that purpose, but other than this issue it's working amazingly (with kernel 5.7.7 brightness and touchpad are working by default). I leave the request here in case it's a similar and easy fix to other laptops (should be, on the same CPU arch).

 

Thanks in advance!

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13 Posts

06-29-2020

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Re:Ideapad 15ARE05 - TSC broken

2020-07-11, 17:24 PM

I'm not sure exactly what the consequences of this are, but I can confirm that the bug is there, and indeed on the latest BIOS, too!

 

Journalctl confirms CPU#0 to CPU#1 timing is off. 

 

Could this cause freezing in low-latency kernels?

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127 Posts

12-17-2018

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Re:Ideapad 15ARE05 - TSC broken

2020-07-13, 10:43 AM

The main issue of using the HPET clock over the TSC clock is the time it takes to return the value from the hardware. The HPET clock although from some opinions is more accurate it is slower to read the data and will cause performance issues on any application that does a call like gettimeofday repeatedly.  I know that those using STEAM are affected which was one of the original complaints but if you are doing general activities you should not have issue with the system using the HPET clock

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07-09-2020

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Re:Ideapad 15ARE05 - TSC broken

2020-07-13, 11:30 AM

@ DaveOber indeed, thank you for acknowledging the issue. My application is very latency sensitive and multiple threads need to grab the current time quite frequently, for this reason performance has been severely impacted. I benchmarked it against another system with TSC and a gettimeofday call is 100x slower on HPET. I am unaware if this can indirectly impact other low level functionality such as context switching and thread scheduling by the OS, but I could see it being the case.

 

In general use, as you said, I do not see any impact, evidenced also by well-known performance benchmark applications.

 

Obviously, giving the user a chance to switch to TSC would be the optimal solution here. I hope Lenovo can justify the time and effort to fix it.

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Re:Ideapad 15ARE05 - TSC broken

2020-07-13, 11:44 AM

Context switching based on interrupts or signal events I would not expect to see much of an issue but if you write a scheduler and use time of day calls to determine the next scheduler wakeup event it should certainly be affected, If you can post the BIOS this system uses for reference I can see if this is an open item to that BIOS team or see if I can add it to their bug list since the fix has already been applied to several different system BIOS releases it may be something they would be willing to consider even if they do not fully support Linux on that system.

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Re:Ideapad 15ARE05 - TSC broken

2020-07-13, 11:49 AM

Much appreciated, the BIOS version I've tested is E7CN24WW , @BrainP reported the same on E7CN25WW .

 

It seems there is a new one today on https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/5-series/ideapad-5-15are05/downloads/DS544050 but the changelog doesn't mention any fixes to TSC.

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