Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention

From: Waiman Long <waiman.long@hpe.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	John Stultz <john.stultz@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Borislav Petkov <bp@suse.de>,
	Jiang Liu <jiang.liu@linux.intel.com>,
	Randy Wright <rwright@hpe.com>,
	Scott J Norton <scott.norton@hpe.com>,
	Douglas Hatch <doug.hatch@hpe.com>,
	Prarit Bhargava <prarit@redhat.com>, X86 ML <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention
Date: Fri, 12 Aug 2016 17:32:01 -0400	[thread overview]
Message-ID: <57AE4051.5060904@hpe.com> (raw)
In-Reply-To: <57AE3C99.8070000@intel.com>

On 08/12/2016 05:16 PM, Dave Hansen wrote:
> On 08/12/2016 01:18 PM, Andy Lutomirski wrote:
>> I don't think this is right.  If the HPET ever returns the same value
>> twice in a row (unlikely because it's generally too slow to read, but
>> it's plausible that someone will make a fast HPET some day), then this
>> could deadlock.
> True...
>
> I guess that means we've got to do some kind of sequence counter
> preferably in the same cacheline as the HPET value itself, or _something
> that we guarantee to change on each write to the cached value.

I have done something similar in the v5 patch that I just sent out.

>> Also, does this code need to be NMI-safe?  This implementation is
>> deadlocky if it's called from an NMI.
> Urg.  Can't we just do
>
> 	if (in_nmi())
> 		return read_real_hpet();
>
> ?

Yes, I am doing that in my v5 patch.

>> The original code was wait-free, right?  That was a nice property, too.
> You mean no spins?  I don't think this one really spins ever either.
>

In the contended case, the reader needs to wait until the new HPET value 
is available. I consider this a kind of waiting.

Cheers,
Longman