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POMeasuring execution time of a function inside linux kernel
 

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  1. POMeasuring execution time of a function inside linux kernel
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    <p>I am using Linux Security Module hooks to add some custom functionality to recv() system call. I want to measure the overhead of this functionality as compared to the pristine recv(). I have written a simple tcp server that I run with and without my module. This tcp server calls a recv() function 'N' number of times. It measures time taken for each recv with something like:</p> <pre><code>clock_gettime(before); recv() clock_gettime(after); global_time += after - before. </code></pre> <p>In the end, I print the average time for a single recv() with "global_time/N". Lets call this time as "user_space_avg_recv" time.</p> <p>Inside my module, I want to place time measurement functions to calculate exact execution time of my hook. I tried 3 methods.</p> <ol> <li><p>I used jiffies as follows:</p> <pre><code>sj = jiffies; my_hook(); ej = jiffies; current-&gt;total_oh = ej - sj; </code></pre> <p>But I see that there is no difference between sj and ej values. Hence total_oh is unchanged.</p></li> <li><p>I used current_kernel_time() since I thought it returns the time in nanoseconds. However, once again, there was no difference in before and after time.</p></li> <li><p>I used get_cycles. I print the total cycles when the process exits. However, when I convert that total cycles values to milliseconds, it comes out be much greater than "user_space_avg_recv" value. This does not make sense as measured value inside kernel always be lesser than the time value measured from user space. This could mean that I am either not measuring using correct API or I am making a mistake while converting value from cycles to milliseconds.</p></li> </ol> <p>I am basically using following formula to convert cycles to milliseconds:</p> <pre><code>avg overhead of my hook in milliseconds = (((cycles / 2.99) / 10^6) / N) </code></pre> <p>2.99 because my clock frequency is 2.99Ghz</p> <p>Some points:</p> <ul> <li><p>My user space program is tied to a single core using set affinity.</p></li> <li><p>I am using kernel 2.6.22.14</p></li> <li><p>To stop kernel from switching contexts while inside my hook, I use preempt_disable() and preempt_enable(). Thus it will not count the execution times of other kernel threads. Even then, since my hook use some I/O, my thread might release the control voluntarily or some interrupt might occur that might increase the total cycles count.</p></li> </ul> <p>Question: How can I measure function execution times accurately inside the kernel?</p>
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