_spin_unlock_irqrestore()在我的kvm中具有很高的采样率，为什么? [英] _spin_unlock_irqrestore() has very high sampling rate in my kvm, why?

问题描述

我在KVM虚拟机中运行SPECJbb基准测试.它显示了仓库2和仓库3之间的吞吐量急剧下降(它们之间的区别只是增加了并发任务)

I run a SPECJbb benchmark in my KVM virtual machine. It shows a drastic drop on throughput between Warehouse 2 and Warehouse 3(The different between them is just addding on cocurrent task)

然后，我在来宾虚拟机中使用perf.它表明_spin_unlock_irqrestore具有很高的采样率.

Then I use perf in my guest virtual machine. It shows that _spin_unlock_irqrestore has very high sampling rate.

事件:31K周期

• 74.89％[内核] [k] _spin_unlock_irqrestore

• 74.89% [kernel] [k] _spin_unlock_irqrestore

7.36％perf-1968.map [.] 0x7f84b913e064

7.36% perf-1968.map [.] 0x7f84b913e064

6.82％[内核] [k] __do_softirq

6.82% [kernel] [k] __do_softirq

6.39％[内核] [k] handle_IRQ_event

6.39% [kernel] [k] handle_IRQ_event

...

运行我的Java程序似乎只占7.36％cpu的时间.为什么_spin_unlock_irqrestore的采样率如此之高?它是做什么的?

It seems that only 7.36% cpu time running my Java program. Why _spin_unlock_irqrestore's sampling rate is so high? And what does it do?

推荐答案

perf的报告不正确，而不是_spin_unlock_irqrestore消耗的周期.

It's bad reporting by perf, not cycles consumed by _spin_unlock_irqrestore.

禁用IRQ时，不会处理性能中断.而是在重新启用中断时对它们进行处理.当perf的中断处理程序查看指令指针时，要查看正在运行的代码，它会找到启用中断的函数-通常是_spin_unlock_irqrestore.

When IRQs are disabled, perf's interrupts are not processed. Instead, they're processed when interrupts are re-enabled. When perf's interrupt handler looks at the instruction pointer, to see what code was running, it finds the function that enabled interrupts - quite often it's _spin_unlock_irqrestore.

因此，您所知道的是，这些周期被禁用了中断的代码占用，并使用_spin_unlock_irqrestore启用了它们.

So all you know is that the cycles were consumed by code that had interrupts disabled, and enabled them using _spin_unlock_irqrestore.

如果您可以使用NMI(不可屏蔽中断)来获得性能，则可以解决此问题.
我知道可以通过更改makefile来使用oprofile(perf的前身)完成，但不了解perf.

If you can get perf to use NMI (non maskable interrupt), it could solve this problem.
I know that it can be done with oprofile (perf's predecessor) by changing the makefile, but don't know about perf.

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