I use the unigine benchmarks sometimes. unigine-heaven and unigine-superposition (which btw is just cool to watch in my opinion). They provide linux packages here benchmark.unigine.com , check your distributions repos too though, some include these too though it’s rare. They are not open source, but games usually aren’t either.
The phoronix one that someone else posted also looks cool, I’ll have to try that one out next time I need something like this.