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

[Waiman Long <longman@redhat.com>]

On 10/4/23 07:50, Ingo Molnar wrote:
> * Waiman Long <longman@redhat.com> 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!

-Longman