Why does opening /proc/cpuinfo take 17 ms?

Question

Running this

strace -ff -tt perl -e 'open(A,"<","/proc/cpuinfo");'

gives this:

22:36:10.365187 openat(AT_FDCWD, "/proc/cpuinfo", O_RDONLY) = 3
22:36:10.382061 ioctl(3, TCGETS, 0x7fffb127ea50) = -1 ENOTTY (Inappropriate ioctl for device)

/proc/cpuinfo is an in-memory file that does not require access to a slow disk. Why is it taking 17 ms to open?

I have tested 10 times on an idle system and the timing is very consistent.

$ uname -a
Linux hk 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Answer 1

从内核的 4.12 版开始，在支持APERF / MPERF MSR 的x86 CPU 上，打开时会有轻微的延迟，/proc/cpuinfo以确保准确报告 CPU 频率（或者至少不要显示太陈旧的值）。这在cpufreq 中略有改变：x86：使 scaling_cur_freq 的行为更符合预期（4.13，延迟增加），再次在x86/CPU：始终在 /proc/cpuinfo 中显示当前 CPU 频率（4.14，延迟减少）。