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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 4 Oct 2023 13:50:16 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Waiman Long <longman@redhat.com>
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>
Subject: Re: [PATCH v6 0/4] x86/speculation: Disable IBRS when idle
Message-ID: <ZR1RePXx6/zZueI8@gmail.com>
References: <20230727184600.26768-1-longman@redhat.com>
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* 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!

	Ingo