Messages in this thread | | | Date | Sun, 18 Jun 2023 21:18:41 -0400 | Subject | Re: [PATCH 0/5] x86/speculation: Disable IBRS when idle | From | Waiman Long <> |
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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
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