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Date:   Mon, 27 Jan 2020 09:36:32 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v15] x86/split_lock: Enable split lock detection by kernel
Message-ID: <20200127083632.GI14946@hirez.programming.kicks-ass.net>
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On Mon, Jan 27, 2020 at 09:04:19AM +0100, Peter Zijlstra wrote:
> On Sat, Jan 25, 2020 at 01:50:03PM -0800, Luck, Tony wrote:
> > On Sat, Jan 25, 2020 at 04:25:25PM -0500, Arvind Sankar wrote:
> > > On Fri, Jan 24, 2020 at 06:47:27PM -0800, Luck, Tony wrote:
> > > > I did find something with a new test. Applications that hit a
> > > > split lock warn as expected. But if they sleep before they hit
> > > > a new split lock, we get another warning. This is may be because
> > > > I messed up when fixing a PeterZ typo in the untested patch.
> > > > But I think there may have been bigger problems.
> > > > 
> > > > Context switch in V14 code did: 
> > > > 
> > > >        if (tifp & _TIF_SLD)
> > > >                switch_to_sld(prev_p);
> > > > 
> > > > void switch_to_sld(struct task_struct *prev)
> > > > {
> > > >        __sld_msr_set(true);
> > > >        clear_tsk_thread_flag(prev, TIF_SLD);
> > > > }
> > > > 
> > > > Which re-enables split lock checking for the next process to run. But
> > > > mysteriously clears the TIF_SLD bit on the previous task.
> > > 
> > > Did Peter mean to disable it only for the current timeslice and
> > > re-enable it for the next time its scheduled?
> > 
> > He's seen and commented on this thread since I made this comment. So
> 
> Yeah, I sorta don't care either way :-)

Part of the reason I did that was to get the MSR back to enabled ASAP,
to limit the blind spot on the sibling.

By leaving the TIF_SLD cleared for a task, and using the XOR logic used
for other TIF flags, the blind spots will be much larger.