Date: Wed, 11 Feb 2015 13:07:24 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
To: Jesper Dangaard Brouer <brouer@redhat.com>
cc: akpm@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        penberg@kernel.org, iamjoonsoo@lge.com
Subject: Re: [PATCH 2/3] slub: Support for array operations
In-Reply-To: <20150211174817.44cc5562@redhat.com>
Message-ID: <alpine.DEB.2.11.1502111305520.7547@gentwo.org>
References: <20150210194804.288708936@linux.com> <20150210194811.902155759@linux.com> <20150211174817.44cc5562@redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Wed, 11 Feb 2015, Jesper Dangaard Brouer wrote:

> > +
> > +
> > +	spin_lock_irqsave(&n->list_lock, flags);
>
> This is quite an expensive lock with irqsave.

Yes but we take it for all partial pages.

> Yet another lock cost.

Yup the page access is shared but there is one per page. Contention is
unlikely.

> > +	spin_unlock_irqrestore(&n->list_lock, flags);
> > +	return allocated;
>
> I estimate (on my CPU) the locking cost itself is more than 32ns, plus
> the irqsave (which I've also found quite expensive, alone 14ns).  Thus,
> estimated 46ns.  Single elem slub fast path cost is 18-19ns. Thus 3-4
> elem bulking should be enough to amortized the cost, guess we are still
> good :-)

We can require that interrupt are off when the functions are called. Then
we can avoid the "save" part?

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