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

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Date:   Fri, 16 Jun 2023 22:14:52 +0200
From:   "Robin Jarry" <rjarry@...hat.com>
To:     "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>
Cc:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        "Joe Mario" <jmario@...hat.com>
Subject: Re: [PATCH 0/5] x86/speculation: Disable IBRS when idle

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>