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

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Message-ID: <946e6723-9635-1db1-d636-0a7904c40d40@redhat.com>
Date:   Wed, 4 Oct 2023 12:11:04 -0400
From:   Waiman Long <longman@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Robin Jarry <rjarry@...hat.com>, Joe Mario <jmario@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v6 0/4] x86/speculation: Disable IBRS when idle

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