linux-kernel - Re: [PATCH v5 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

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Date:   Wed, 26 Jul 2023 12:25:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     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>
Subject: Re: [PATCH v5 4/4] intel_idle: Add ibrs_off module parameter to
 force disable IBRS

On Tue, Jul 25, 2023 at 04:45:49PM -0400, Waiman Long wrote:
> On 7/10/23 15:48, Waiman Long wrote:
> > Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
> > disables IBRS when the cstate is 6 or lower. However, there are
> > some use cases where a customer may want to use max_cstate=1 to
> > lower latency. Such use cases will suffer from the performance
> > degradation caused by the enabling of IBRS in the sibling idle thread.
> > Add a "ibrs_off" module parameter to force disable IBRS and the
> > CPUIDLE_FLAG_IRQ_ENABLE flag if set.
> > 
> > In the case of a Skylake server with max_cstate=1, this new ibrs_off
> > option will likely increase the IRQ response latency as IRQ will now
> > be disabled.
> > 
> > When running SPECjbb2015 with cstates set to C1 on a Skylake system.
> > 
> > First test when the kernel is booted with: "intel_idle.ibrs_off"
> >    max-jOPS = 117828, critical-jOPS = 66047
> > 
> > Then retest when the kernel is booted without the "intel_idle.ibrs_off"
> > added.
> >    max-jOPS = 116408, critical-jOPS = 58958
> > 
> > That means booting with "intel_idle.ibrs_off" improves performance by:
> >    max-jOPS:   1.2%, which could be considered noise range.
> >    critical-jOPS: 12%, which is definitely a solid improvement.
> > 
> > The admin-guide/pm/intel_idle.rst file is updated to add a description
> > about the new "ibrs_off" module parameter.
> > 
> > Signed-off-by: Waiman Long <longman@...hat.com>
> > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >   Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
> >   drivers/idle/intel_idle.c                   | 11 ++++++++++-
> >   2 files changed, 26 insertions(+), 2 deletions(-)
> 
> Ping! Is there further suggested changes for this patch series or is it good
> enough to be merged?

I suppose I'll go stick it in x86/core unless someone objects.