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Subject: Re: [profile] fix timer interrupt livelock on 512x Altix
From: Rusty Russell <rusty@rustcorp.com.au>
To: William Lee Irwin III <wli@holomorphy.com>
Cc: Andrew Morton <akpm@osdl.org>, John Hawkes <hawkes@sgi.com>,
       Ray Bryant <raybry@sgi.com>, Jesse Barnes <jbarnes@engr.sgi.com>,
       lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <20040920213020.GX9106@holomorphy.com>
References: <20040920213020.GX9106@holomorphy.com>
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Date: Fri, 15 Oct 2004 11:46:22 +1000
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On Tue, 2004-09-21 at 07:30, William Lee Irwin III wrote:
> +static void __profile_flip_buffers(void *unused)
> +{
> +	int cpu = smp_processor_id();
> +
> +	per_cpu(cpu_profile_flip, cpu) = !per_cpu(cpu_profile_flip, cpu);
> +}

Please: one point of per-cpu vars is that archs can choose to hold the
per-cpu offset in a reg, and derive smp_processor_id() from that.  By
doing the reverse, manually, you defeat this.  How about:

	int *flip = __get_cpu_var(cpu_profile_flip);
	*flip = !*flip;

> +
> +static void profile_flip_buffers(void)
> +{
> +	int i, j, cpu;
> +
> +	down(&profile_flip_mutex);
> +	j = per_cpu(cpu_profile_flip, get_cpu());
> +	put_cpu();

Similarly, this is equivalent to:
	j = __get_cpu_var(cpu_profile_flip);

Cheers,
Rusty.
-- 
Anyone who quotes me in their signature is an idiot -- Rusty Russell