I want to know a simple method to calculate running task execution time. Is there any FREE RTOS API can do this?

There are several ways of doing this, but in each case you need to supply a fast clock as a time base. The fast clock needs to provide time at a rate many times higher than the tick count increment, otherwise you will not get the resolution you need.

If you are using a Cortex-M then a convenient method is to use the cycle counter from the debug circuitry - if you are using Cortex-M (would have been easier if you had stated which chip you were using in your original question) then let me know and I can point you in the direction of the appropriate code. Otherwise here is a generic page on the subject:

http://www.freertos.org/rtos-run-time-stats.html

In addition you can define the traceTASKSWITCHEDIN or traceTASKSWITCHEDOUT macros to do whatever you want, including keeping your own run time date. Finally you could the FreeRTOS+Trace tool.

Regards.

I am using Cortex-M3 (Atmel SAM3U4E). FreeRTOS Resolution of 1 ms is very small to what I want to measure. It will be good if I have a resolution of 50-100 Micro Sec if possible.

You can use the run-time stats implementation found in the SAM4E demo. Look at the file RunTimeStatsTimer.c in FreeRTOSDemoCORTEXM4ATSAM4EAtmelStudiosrc, and the run time stats related FreeRTOSConfig.h settings in FreeRTOSDemoCORTEXM4ATSAM4EAtmelStudiosrcconfigFreeRTOSConfig.h.

Regards.