linux-kernel - Re: [PATCH tip/core/rcu 19/19] rcu: Add fastpath bypassing funnel locking

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Date:	Thu, 30 Jul 2015 16:44:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	dave@...olabs.net, waiman.long@...com
Subject: Re: [PATCH tip/core/rcu 19/19] rcu: Add fastpath bypassing funnel
 locking

On Fri, Jul 17, 2015 at 04:29:24PM -0700, Paul E. McKenney wrote:

>  	/*
> +	 * First try directly acquiring the root lock in order to reduce
> +	 * latency in the common case where expedited grace periods are
> +	 * rare.  We check mutex_is_locked() to avoid pathological levels of
> +	 * memory contention on ->exp_funnel_mutex in the heavy-load case.
> +	 */
> +	rnp0 = rcu_get_root(rsp);
> +	if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {
> +		if (mutex_trylock(&rnp0->exp_funnel_mutex)) {
> +			if (sync_exp_work_done(rsp, rnp0, NULL,
> +					       &rsp->expedited_workdone0, s))
> +				return NULL;
> +			return rnp0;
> +		}
> +	}

So our 'new' locking primitives do things like:

static __always_inline int queued_spin_trylock(struct qspinlock *lock)
{
        if (!atomic_read(&lock->val) &&
           (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) == 0))
                return 1;
        return 0;
}

mutexes do not do this.

Now I suppose the question is, does that extra read slow down the
(common) uncontended case? (remember, we should optimize locks for the
uncontended case, heavy lock contention should be fixed with better
locking schemes, not lock implementations).

Davidlohr, Waiman, do we have data on this?

If the extra read before the cmpxchg() does not hurt, we should do the
same for mutex and make the above redundant.
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