Linux 6.2's Call Depth Tracking Helps Recover Lost Performance On Intel Skylake CPUs

Right off the bat with the PostMark I/O benchmark there was a clear improvement with retbleed=stuff over the default, out-of-the-box kernel performance where IBRS is activated on Skylake. Call Depth Tracking yielded a 24% improvement over the default performance in PostMark, but still the Call Depth Tracking mode had around 84% the performance of running without any Retbleed mitigations or 69% the performance when running the system without any CPU security mitigations.

For the Sockperf network socket API benchmark there was also clear benefit to Call Depth Tracking over the default performance on Linux 6.2, but still all the other mitigations do incur significant overhead.

But if right now you are running Linux out-of-the-box on a Skylake era system and using the default security mitigations that impose IBRS, with Linux 6.2+ switching over to Call Depth Tracking can at least recover some of that lost performance.