Re: [PATCH 0/5] x86/speculation: Disable IBRS when idle

[Waiman Long <longman@redhat.com>]

On 6/17/23 08:21, Peter Zijlstra wrote:
> On Fri, Jun 16, 2023 at 10:14:52PM +0200, Robin Jarry wrote:
>> Waiman Long, Jun 16, 2023 at 21:59:
>>> 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.
>>>
>>> This patch series turns off IBRS when in various idle mode to eliminate
>>> the performance impact of the idling thread on its busy sibling thread.
>> Hi Longman,
>>
>> thanks a lot for the quick turnaround on this issue.
>>
>> Tested-by: Robin Jarry <rjarry@redhat.com>
> I can't see the patches -- they didn't arrive in my mailbox nor can I
> find them in the archive, in fact this here mail is the only evidence
> they exist at all.

I got a rebound message from your mail server about incorrect message 
format. It is probably caused by some problem in my end.


> However, did you all see intel_idle_ibrs() and how that is selected for
> C6 and up?
>
> What exactly isn't working there?

We were testing on the RHEL9.2 kernel which doesn't have your 
intel_idle_ibrs() patch yet. My preliminary testing does indicate your 
patch will likely work. I will ask Jerry to test a newer RHEL9.3 kernel 
with the intel_idle_ibrs() patch to see if it helps.

> Also, instead of investing more in this IBRS trainwreck, did you all try
> call-depth-stuffing ?

Yes, we are planning to backport your call-depth-stuffing code, but I 
believe there is still some issue outstanding that you need to address. 
So we need a solution to work around this issue in the mean time.

Cheers,
Longman