Message-ID: <538F7C7D.9040606@linux.vnet.ibm.com>
Date: Thu, 05 Jun 2014 01:37:25 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: akpm@linux-foundation.org, fweisbec@gmail.com, tglx@linutronix.de,
        hch@infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] smp, ipi: Speed up IPI handling by invoking the callbacks
 in reverse order
References: <20140604193934.21321.19185.stgit@srivatsabhat.in.ibm.com> <20140604194731.GO13930@laptop.programming.kicks-ass.net>
In-Reply-To: <20140604194731.GO13930@laptop.programming.kicks-ass.net>
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On 06/05/2014 01:17 AM, Peter Zijlstra wrote:
> On Thu, Jun 05, 2014 at 01:09:35AM +0530, Srivatsa S. Bhat wrote:
>> The current implementation of lockless list (llist) has a drawback: if we
>> want to traverse the list in FIFO order (oldest to newest), we need to
>> reverse the list first (and this can be expensive if the list is large,
>> since this is an O(n) operation).
> 
> Have you actually looked at the queue depth of this thing? Large queues
> are a problem for interrupt latency.
> 

Actually, I wrote this patch just by looking at the code and realizing
that we don't need to reverse the list. In practice, I haven't actually
seen any noticeable interrupt latencies or large queues so far. So I think
this patch is just a very tiny optimization, that's all.

Regards,
Srivatsa S. Bhat

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