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

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Date:	Fri, 12 Aug 2016 17:32:01 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Hansen <dave.hansen@...el.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	Randy Wright <rwright@....com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Prarit Bhargava <prarit@...hat.com>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention

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