netdev - Re: net: Utility function to get affinity

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Message-ID: <20140306222441.GA17512@mtl-eit-vdi-22.mtl.labs.mlnx>
Date:	Fri, 7 Mar 2014 00:24:43 +0200
From:	Amir Vadai <amirv@...lanox.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, yevgenyp@...lanox.com,
	ogerlitz@...lanox.com, prarit@...hat.com, gvaradar@...co.com
Subject: Re: net: Utility function to get affinity_hint by policy

On 06/03/14 15:38 -0500, David Miller wrote:
> From: Amir Vadai <amirv@...lanox.com>
> Date: Thu,  6 Mar 2014 19:18:51 +0200
> 
> > +	rxq %= num_online_cpus();
> > +
> > +	p_numa_cores_mask = cpumask_of_node(numa_node);
> > +	if (!p_numa_cores_mask)
> > +		p_numa_cores_mask = cpu_online_mask;
> > +
> > +	for_each_cpu(affinity_cpu, p_numa_cores_mask) {
> > +		if (--rxq < 0)
> > +			goto out;
> > +	}
> > +
> > +	if (!zalloc_cpumask_var(&non_numa_cores_mask, GFP_KERNEL)) {
> > +		ret = -ENOMEM;
> > +		goto err;
> > +	}
> > +
> > +	cpumask_xor(non_numa_cores_mask, cpu_online_mask, p_numa_cores_mask);
> > +
> > +	for_each_cpu(affinity_cpu, non_numa_cores_mask) {
> > +		if (--rxq < 0)
> > +			goto out;
> > +	}
> > +
> > +	ret = -EINVAL;
> > +	goto err;
> 
> The NUMA core mask and the non-NUMA core mask must together provide
> at least num_online_cpus() bits.
> 
> Said another way, since rxq is limited to num_online_cpus() it should
> never exceed local + non-local cores.
> 
> Therefore this -EINVAL path seems like it should not be possible, and
> therefore perhaps a WARN_ON() or similar would be appropriate.

All these could be changed dynamically, and since there is no
protection on this block of code, it is possible that local + non-local
at point of time t1 will exceed num_online_cpus() at t2.
Since this is very rare and the fix for that is very complicated, I
decided not set affinity hint in this scenario, and only return an
error code.

Amir
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