Date: Tue, 11 Nov 2003 13:59:15 +0530
From: Ravikiran G Thirumalai <kiran@in.ibm.com>
To: Jack Steiner <steiner@sgi.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: hot cache line due to note_interrupt()
Message-ID: <20031111082915.GC1130@llm08.in.ibm.com>
References: <20031110215844.GC21632@sgi.com>
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On Mon, Nov 10, 2003 at 03:58:44PM -0600, Jack Steiner wrote:
> 
> I dont know the background on note_interrupt() in arch/ia64/kernel/irq.c, 
> but I had to disable the function on our large systems (IA64).
> 
> The function updates a counter in the irq_desc_t table. An entry in this table
> is shared by all cpus that take a specific interrupt #. For most interrupt #'s,
> this is a problem but it is prohibitive for the timer tick on big systems.
> 
> Updating the counter causes a cache line to be bounced between
> cpus at a rate of at least HZ*active_cpus. (The number of bus transactions

The answer to this is probably alloc_percpu for the counters.
right now this might not possible because irq_desc_t table might be used very
early, and alloc_percpu uses slab underneath.  alloc_percpu will have to be 
made to work early enough for this....

Thanks,
Kiran

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