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Message-ID: <946e6723-9635-1db1-d636-0a7904c40d40@redhat.com>
Date:   Wed, 4 Oct 2023 12:11:04 -0400
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Subject: Re: [PATCH v6 0/4] x86/speculation: Disable IBRS when idle
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To:     Ingo Molnar <mingo@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Len Brown <lenb@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        x86@kernel.org, linux-pm@vger.kernel.org,
        Robin Jarry <rjarry@redhat.com>, Joe Mario <jmario@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <20230727184600.26768-1-longman@redhat.com>
 <ZR1RePXx6/zZueI8@gmail.com>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <ZR1RePXx6/zZueI8@gmail.com>
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On 10/4/23 07:50, Ingo Molnar wrote:
> * Waiman Long <longman@redhat.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