linux-kernel - Re: [PATCH] x86/platform/uv/BAU: disable BAU on single hub configurations

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Date:   Thu, 20 Jul 2017 13:47:50 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andrew Banman <abanman@....com>
Cc:     mingo@...hat.com, tglx@...utronix.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, rja@....com, mike.travis@....com
Subject: Re: [PATCH] x86/platform/uv/BAU: disable BAU on single hub
 configurations


* Andrew Banman <abanman@....com> wrote:

> The BAU confers no benefit to a UV system running with only one hub/socket.
> Permanently disable the BAU driver if there are less than two hubs online
> to avoid BAU overhead. We have observed failed boots on single-socket UV4
> systems caused by BAU that are avoided with this patch.
> 
> Signed-off-by: Andrew Banman <abanman@....com>
> Acked-by: Russ Anderson <rja@....com>
> Acked-by: Mike Travis <mike.travis@....com>
> ---
>  arch/x86/platform/uv/tlb_uv.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/platform/uv/tlb_uv.c b/arch/x86/platform/uv/tlb_uv.c
> index 2511a28..88216cc 100644
> --- a/arch/x86/platform/uv/tlb_uv.c
> +++ b/arch/x86/platform/uv/tlb_uv.c
> @@ -2251,6 +2251,12 @@ static int __init uv_bau_init(void)
>  	}
>  
>  	nuvhubs = uv_num_possible_blades();
> +	if (nuvhubs < 2) {
> +		pr_crit("UV: BAU disabled - insufficient hub count\n");
> +		set_bau_off();
> +		nobau_perm = 1;
> +		return 0;
> +	}

Yeah, could you structure the error paths in this function in a bit more organized 
fashion? It has two similar error handling blocks:


                pr_crit("UV: BAU disabled - insufficient hub count\n");
                set_bau_off();
                nobau_perm = 1;
                return 0;

...

                set_bau_off();
                nobau_perm = 1;
                return 0;

which could be consolidated via the usual goto exception construct:

	if (nuvhubs < 2) {
		pr_crit("UV: BAU disabled - insufficient hub count\n");
		goto err_disable_bau;
	}
	...

        if (init_per_cpu(nuvhubs, uv_base_pnode))
		pr_crit("UV: BAU disabled - per CPU init failed\n");
		goto err_disable_bau;
	}

	...
	return 0;

err_disable_bau:

	set_bau_off();
	nobau_perm = 1;
	return 0;

Note that I added an error message to the second case as well.

Plus, in the error case you might want to use a 'return -EINVAL;' instead of 
return 0, or so?

Plus plus, there's probably a (mild) memory leak in the error paths, can the 
cpumasks be free_cpumask_var() freed - or are they still required even if BAU is 
disabled?

Thanks,

	Ingo