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

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Date:	Fri, 12 Aug 2016 10:16:25 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Waiman Long <waiman.long@....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 08/12/2016 10:01 AM, Waiman Long wrote:
> The reason for using a special lock is that I want both sequence number
> update and locking to be done together atomically. They can be made
> separate as is in the seqlock. However, that will make the code more
> complex to make sure that all the threads see a consistent set of lock
> state and sequence number.

Why do we need a sequence number?  The "cached" HPET itself could be used.

I'm thinking something like below could use a spinlock instead of the
doing a custom cmpxchg sequence.  The spin_is_locked() should allow the
contended "readers" to avoid using atomics.

spinlock_t hpet_lock;
u32 hpet_value;
...
{
	u32 old_hpet = READ_ONCE(hpet_value);
	u32 new_hpet;

	// need to ensure that the spin_is_locked() is ordered after
	// the READ_ONCE().
	smp_rmb();
	// spin_is_locked() doesn't do atomics
	if (!spin_is_locked(&hpet_lock) && spin_trylock(&hpet_lock)) {
		WRITE_ONCE(hpet_value, real_read_hpet());
		spin_unlock(&hpet_lock);
		return hpet_value;
	}
	// Contended case.  We spin here waiting for the guy who holds
	// the lock to write a new value to 'hpet_value'.
	//
	// We know that our old_hpet is older than our check for the
	// spinlock being locked. So, someone must either have already
	// updated it or be updating it.
	do {
		cpu_relax();
		// We do not do a rmb() here.  We don't need a guarantee
		// that this read is up-to-date, just that it will
		// _eventually_ see an up-to-date value.
		new_hpet = READ_ONCE(hpet_value);
	} while (old_hpet == new_hpet);
	return new_hpet;
}