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Subject: Re: [PATCH 1/2] softirq: fix integer overflow in function show_stat()
To:     Matthew Wilcox <willy@infradead.org>
Cc:     "Paul E . McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Zhen Lei <thunder.leizhen@huawei.com>
References: <20230724132224.916-1-thunder.leizhen@huaweicloud.com>
 <20230724132224.916-2-thunder.leizhen@huaweicloud.com>
 <ZL6BwiHhvQneJZYH@casper.infradead.org>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huaweicloud.com>
Message-ID: <6e38e31f-4413-1aff-8973-5c3d660bedea@huaweicloud.com>
Date:   Tue, 25 Jul 2023 10:00:38 +0800
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On 2023/7/24 21:50, Matthew Wilcox wrote:
> On Mon, Jul 24, 2023 at 09:22:23PM +0800, thunder.leizhen@huaweicloud.com wrote:
>> From: Zhen Lei <thunder.leizhen@huawei.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).

Yes, I think patch 2/2 can be dropped. I reduced the number of soft
interrupts generated in one second, and actually 100+ or 1000 is normal.
But I think patch 1/2 is necessary. The sum of the output scattered values
does not match the output sum. To solve this problem, we only need to
adjust the type of a local variable.


> 
> .
> 

-- 
Regards,
  Zhen Lei