linux-kernel - Re: [PATCH v1 2/3] locking/pvqspinlock, hv: Enable PV qspinlock for Hyper-V

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Message-ID: <20180917125452.GC11769@yi.y.sun>
Date:   Mon, 17 Sep 2018 20:54:52 +0800
From:   Yi Sun <yi.y.sun@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        chao.p.peng@...el.com, chao.gao@...el.com,
        isaku.yamahata@...el.com, michael.h.kelly@...rosoft.com,
        tianyu.lan@...rosoft.com, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH v1 2/3] locking/pvqspinlock, hv: Enable PV qspinlock for
 Hyper-V

On 18-09-14 12:42:33, Thomas Gleixner wrote:
> On Fri, 14 Sep 2018, Yi Sun wrote:
> > > > +static void hv_notify_long_spin_wait(void)
> > > > +{
> > > > +	u64 input = spin_wait_info | 0x00000000ffffffff;
> > > 
> > > What? The result is always 0x00000000ffffffff .....
> > > 
> > Oh, sorry for such error.
> > 
> > > > +	spin_wait_info++;
> > > > +	hv_do_fast_hypercall8(HVCALL_NOTIFY_LONG_SPIN_WAIT, input);
> > > > +}
> > > > +
> > > > +static void hv_qlock_kick(int cpu)
> > > > +{
> > > > +	spin_wait_info--;
> > > 
> > > Looking at the above this is completely pointless and I have no idea what
> > > that variable is supposed to do.
> > > 
> > Per Microsoft Hypervisor Top Level Functional Specification, the input
> > parameter of HVCALL_NOTIFY_LONG_SPIN_WAIT is defined as below:
> > 
> > SpinwaitInfo – Specifies the accumulated count the guest was spinning.
> > 
> > So, it is increased when guest is spinning and reduced when spinlock is
> > released.
> 
> But that's a global variable, so it might be incremented and decremented by
> several CPUs at once. I don't have the spec and have no time to study it
> either, but global does not make any sense to me. The spin wait info comes
> from a single guest CPU and the wakeup is targeted at that guest CPU as
> well. So why global? It might be defined that way, but then you really want
> to explain it proper.
> 
> Thanks,
> 
> 	tglx
> 
Let me check this more. Will reply to you later. Thanks!

BRs,
Yi Sun