linux-kernel - Re: [PATCH 1/2] softirq: fix integer overflow in function show

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Date:   Mon, 24 Jul 2023 14:50:58 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     thunder.leizhen@...weicloud.com
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Zhen Lei <thunder.leizhen@...wei.com>
Subject: Re: [PATCH 1/2] softirq: fix integer overflow in function show_stat()

On Mon, Jul 24, 2023 at 09:22:23PM +0800, thunder.leizhen@...weicloud.com wrote:
> From: Zhen Lei <thunder.leizhen@...wei.com>
> 
> The statistics function of softirq is supported by commit aa0ce5bbc2db
> ("softirq: introduce statistics for softirq") in 2009. At that time,
> 64-bit processors should not have many cores and would not face
> significant count overflow problems. Now it's common for a processor to
> have hundreds of cores. Assume that there are 100 cores and 10
> TIMER_SOFTIRQ are generated per second, then the 32-bit sum will be
> overflowed after 50 days.

50 days is long enough to take a snapshot.  You should always be using
difference between, not absolute values, and understand that they can
wrap.  We only tend to change the size of a counter when it can wrap
sufficiently quickly that we might miss a wrap (eg tens of seconds).