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

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Date:   Sat, 17 Jun 2023 14:21:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Robin Jarry <rjarry@...hat.com>
Cc:     Waiman Long <longman@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org> 
        , Josh Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta
        <pawan.kumar.gupta@...ux.intel.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
        <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben
        Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel
        Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider" 
        <vschneid@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH 0/5] x86/speculation: Disable IBRS when idle

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@...hat.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.

However, did you all see intel_idle_ibrs() and how that is selected for
C6 and up?

What exactly isn't working there?

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