linux-kernel - Re: [PATCH v4 6/7] sched/topology: Introduce for_each_numa_hop

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Date:   Tue, 27 Sep 2022 17:45:21 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Tony Luck <tony.luck@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Gal Pressman <gal@...dia.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: [PATCH v4 6/7] sched/topology: Introduce for_each_numa_hop_cpu()

On 25/09/22 07:58, Yury Norov wrote:
> On Fri, Sep 23, 2022 at 04:55:41PM +0100, Valentin Schneider wrote:
>> +/**
>> + * for_each_numa_hop_cpu - iterate over CPUs by increasing NUMA distance,
>> + *                         starting from a given node.
>> + * @cpu: the iteration variable.
>> + * @node: the NUMA node to start the search from.
>> + *
>> + * Requires rcu_lock to be held.
>> + * Careful: this is a double loop, 'break' won't work as expected.
>
> This warning concerns me not only because new iteration loop hides
> complexity and breaks 'break' (sic!), but also because it looks too
> specific. Why don't you split it, so instead:
>
>        for_each_numa_hop_cpu(cpu, dev->priv.numa_node) {
>                cpus[i] = cpu;
>                if (++i == ncomp_eqs)
>                        goto spread_done;
>        }
>
> in the following patch you would have something like this:
>
>        for_each_node_hop(hop, node) {
>                struct cpumask hop_cpus = sched_numa_hop_mask(node, hop);
>
>                for_each_cpu_andnot(cpu, hop_cpus, ...) {
>                        cpus[i] = cpu;
>                        if (++i == ncomp_eqs)
>                                goto spread_done;
>                }
>        }
>
> It looks more bulky, but I believe there will be more users for
> for_each_node_hop() alone.
>
> On top of that, if you really like it, you can implement
> for_each_numa_hop_cpu() if you want.
>

IIUC you're suggesting to introduce an iterator for the cpumasks first, and
then maybe add one on top for the individual cpus.

I'm happy to do that, though I have to say I'm keen to keep the CPU
iterator - IMO the complexity is justified if it is centralized in one
location and saves us from boring old boilerplate code.

>> + * Implementation notes:
>> + *
>> + * Providing it is valid, the mask returned by
>> + *  sched_numa_hop_mask(node, hops+1)
>> + * is a superset of the one returned by
>> + *   sched_numa_hop_mask(node, hops)
>> + * which may not be that useful for drivers that try to spread things out and
>> + * want to visit a CPU not more than once.
>> + *
>> + * To accommodate for that, we use for_each_cpu_andnot() to iterate over the cpus
>> + * of sched_numa_hop_mask(node, hops+1) with the CPUs of
>> + * sched_numa_hop_mask(node, hops) removed, IOW we only iterate over CPUs
>> + * a given distance away (rather than *up to* a given distance).
>> + *
>> + * hops=0 forces us to play silly games: we pass cpu_none_mask to
>> + * for_each_cpu_andnot(), which turns it into for_each_cpu().
>> + */
>> +#define for_each_numa_hop_cpu(cpu, node)				       \
>> +	for (struct { const struct cpumask *curr, *prev; int hops; } __v =     \
>> +		     { sched_numa_hop_mask(node, 0), NULL, 0 };		       \
>
> This anonymous structure is never used as structure. What for you
> define it? Why not just declare hops, prev and curr without packing
> them?
>

I haven't found a way to do this that doesn't involve a struct - apparently
you can't mix types in a for loop declaration clause.

> Thanks,
> Yury
>