* Waiman Long <longman@xxxxxxxxxx> wrote: > For Intel processors that need to turn on IBRS to protect against > Spectre v2 and Retbleed, the IBRS bit in the SPEC_CTRL MSR affects > the performance of the whole core even if only one thread is turning > it on when running in the kernel. For user space heavy applications, > the performance impact of occasionally turning IBRS on during syscalls > shouldn't be significant. Unfortunately, that is not the case when the > sibling thread is idling in the kernel. In that case, the performance > impact can be significant. > > When DPDK is running on an isolated CPU thread processing network packets > in user space while its sibling thread is idle. The performance of the > busy DPDK thread with IBRS on and off in the sibling idle thread are: > > IBRS on IBRS off > ------- -------- > packets/second: 7.8M 10.4M > avg tsc cycles/packet: 282.26 209.86 > > This is a 25% performance degradation. The test system is a Intel Xeon > 4114 CPU @ 2.20GHz. Ok, that's a solid improvement, and the feature has no obvious downsides, so I've applied your series to tip:sched/core with a few edits here and there. Thanks! Ingo
