Re: [PATCH v6 0/4] x86/speculation: Disable IBRS when idle

[Ingo Molnar]

* 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