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

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Date:	Thu, 11 Aug 2016 19:22:00 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Hansen <dave.hansen@...el.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, Jiang Liu <jiang.liu@...ux.intel.com>,
	Borislav Petkov <bp@...e.de>,
	Andy Lutomirski <luto@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Randy Wright <rwright@....com>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention

On 08/11/2016 03:32 PM, Dave Hansen wrote:
> On 08/10/2016 11:29 AM, Waiman Long wrote:
>> +static cycle_t read_hpet(struct clocksource *cs)
>> +{
>> +	int seq;
>> +
>> +	seq = READ_ONCE(hpet_save.seq);
>> +	if (!HPET_SEQ_LOCKED(seq)) {
> ...
>> +	}
>> +
>> +	/*
>> +	 * Wait until the locked sequence number changes which indicates
>> +	 * that the saved HPET value is up-to-date.
>> +	 */
>> +	while (READ_ONCE(hpet_save.seq) == seq) {
>> +		/*
>> +		 * Since reading the HPET is much slower than a single
>> +		 * cpu_relax() instruction, we use two here in an attempt
>> +		 * to reduce the amount of cacheline contention in the
>> +		 * hpet_save.seq cacheline.
>> +		 */
>> +		cpu_relax();
>> +		cpu_relax();
>> +	}
>> +
>> +	return (cycle_t)READ_ONCE(hpet_save.hpet);
>> +}
> It's a real bummer that this all has to be open-coded.  I have to wonder
> if there were any alternatives that you tried that were simpler.

What do you mean by "open-coded"? Do you mean the function can be inlined?


> Is READ_ONCE()/smp_store_release() really strong enough here?  It
> guarantees ordering, but you need ordering *and* a guarantee that your
> write is visible to the reader.  Don't you need actual barriers for
> that?  Otherwise, you might be seeing a stale HPET value, and the spin
> loop that you did waiting for it to be up-to-date was worthless.  The
> seqlock code, uses barriers, btw.

The cmpxchg() and smp_store_release() act as the lock/unlock sequence 
with the proper barriers. Another important point is that the hpet value 
is visible to the other readers  before the sequence number. This is 
what the smp_store_release() is providing. cmpxchg is an actual barrier, 
even though smp_store_release() is not. However, the x86 architecture 
will guarantee the writes are in order, I think.

> Also, since you're fundamentally reading a second-hand HPET value, does
> that have any impact on the precision of the HPET as a timesource?  Or,
> is it so coarse already that this isn't an issue?

There can always be unexpected latency in the returned time value, such 
as an interrupt or NMI. I think as long as the time won't go backward, 
it should be fine.

Cheers,
Longman