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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>
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 <3b1ba209-58c8-b2b6-115a-6c43cba80098@huaweicloud.com>
 <ZL/pvjMMtlxvBSCm@casper.infradead.org>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huaweicloud.com>
Message-ID: <d6f00e09-757a-9935-bc22-c2a1d9c99c52@huaweicloud.com>
Date:   Wed, 26 Jul 2023 08:59:13 +0800
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On 2023/7/25 23:26, Matthew Wilcox wrote:
> On Tue, Jul 25, 2023 at 05:09:05PM +0800, Leizhen (ThunderTown) wrote:
>> On 2023/7/25 10:00, Leizhen (ThunderTown) wrote:
>>> 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).
>>
>> Sometimes it can take a long time to view it again. For example, it is
>> possible to run a complete business test for hours or even days, and
>> then calculate the average.
> 
> I've been part of teams which have done such multi-hour tests.  That
> isn't how monitoring was performed.  Instead snapshots were taken every
> minute or even more frequently, because we wanted to know how these
> counters were fluctuating during the test -- were there time periods
> when the number of sortirqs spiked, or was it constant during the test?
> 
>>> 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.
>>
>> However, it is important to consider that when the local variable is changed
>> to u64, the output string becomes longer. It is not clear if the user-mode
>> program parses it only by u32.
> 
> There's no need for the numbers to add up.  They won't anyway, because
> summing them is racy , so they'll always be a little off.

Okay, thanks for the reply. I got it. I just summed it up temporarily to
prove that integer overflow is possible, and there's no actual requirement.

> 
> .
> 

-- 
Regards,
  Zhen Lei