My macbook has a high cpu usage because of one process and i don't know why.
This is the process all the time at 130% 
I think it has something to do with this log. The log is getting spammed only with this message. 
MacOS – com.apple.bkagentservice high cpu usage
cpumacbook promacos
Related Solutions
By observing log messages in ~/Library/Logs/Ubiquity/<my username>/, I realized that Ubiquity (iCloud process) kept trying to create a file in a folder that doesn't exist:
I opened up the Terminal and created this directory and its parent directories up to .ubd:
cd ~/Library/Mobile\ Documents/
sudo mkdir .ubd && cd .ubd
sudo mkdir peer-E2B13E4F-56F7-C79B-1621-E30738B638FE-v23 && cd peer-E2B13E4F-56F7-C79B-1621-E30738B638FE-v23
sudo mkdir ftr
Then I changed the newly created directories' owner from root to myself:
cd ../..
sudo chown -R <my username> .ubd/
The fan turned silent instantly.
As far as I am concerned kernel_task was always trying to tell me that the computer is running hot. It seems to be a strange countermeasure which occupies the CPU with a low energy No-Op task which prevents it from doing "hotter" things. So you should be looking for that "other" process eating up the rest of your CPU time.
However I noticed very high CPU usage of Safari ever since I update do Yosemite 10.10.4. The browser visibly (in Activity Monitor) starts several processes now and two of them (Safari Networking and nsurlstoraged) routinely take up a lot of CPU, especially Safari Networking often takes more than 50% of one CPU. It seems to be less constant now, so maybe there is some "trickle down" effect on this one.
Best Answer
com.apple.bkagentservice is part of BookKit, which itself is part of iBooks.
Based on the limited information in your question I'd guess the BKAgentService is trying to reconcile your books in iBooks.app on your Macbook with the books registered against your account in iCloud, and some conflict is causing it to go a little mental.
Something I'd try is closing iBooks, killing the BKAgentService, then renaming the following two folders and re-opening iBooks. This will relaunch BKAgentService and it should start to sync your library from scratch.