system_call_after_swapgs, where is my code spending most of the time?

Question

I am trying to profile my code with intel Vtune. When looking at the function call stack it looks like most of the time is spent on a function called system_call_after_swapgs. However there is no stack information. My question is: what is system_call_after_swapgs? And how do I know where it was called from?

It appears to be a Linux (kernel?) system function. If you in fact do not use Linux, you can remove the tag I added. — Vladimir F Героям слава, Mar 21 '18 at 10:25
Do you have any guess about what it is doing? My feeling is that the program spends a lot of time waiting for threads to finish due to a high load imbalance. However I can't see which part of the code is running slow. — Manfredo, Mar 22 '18 at 10:31

score 3 · Answer 1 · edited Apr 26 '18 at 08:19

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I also see that sysret_check is very high. I bet if you run general exploration, then extend frontend bound/frontend latency, you'll see high % of branch resteers. If so, this is likely a performance side effects of u-code patches against Spectre attacks, that clear BTB on system calls/returns to user mode.

If this system is air-gapped, or you have other good protection measures against attacks gaining local shell, you could revert BIOS to pre-2018 versions that do not include the u-code patch.

edited Apr 26 '18 at 08:19

Vladimir F Героям слава

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answered Apr 26 '18 at 08:12

izard

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This comment is the closest I can find to what `sysret_check` IS. Can you expand this answer at all, perhaps with references? – jwm Jun 04 '21 at 23:58

system_call_after_swapgs, where is my code spending most of the time?

1 Answers1