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Subject: Re: [PATCH next] softirq: enable MAX_SOFTIRQ_TIME tuning with sysctl
 max_softirq_time_usecs
To:     Thomas Gleixner <tglx@linutronix.de>
CC:     <corbet@lwn.net>, <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>, <akpm@linux-foundation.org>,
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        <linux@leemhuis.info>, <alex.popov@linux.com>,
        <linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-fsdevel@vger.kernel.org>,
        "wangxiaogang (F)" <wangxiaogang3@huawei.com>,
        "Zhoukang (A)" <zhoukang7@huawei.com>,
        Mingfangsen <mingfangsen@huawei.com>, <tedheadster@gmail.com>,
        Eric Dumazet <edumazet@google.com>
References: <f274f85a-bbb6-3e32-b293-1d5d7f27a98f@huawei.com>
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 <alpine.DEB.2.21.1906241141370.32342@nanos.tec.linutronix.de>
From:   Zhiqiang Liu <liuzhiqiang26@huawei.com>
Message-ID: <c1b7a345-fa22-e52a-4db8-1f1288e7ad15@huawei.com>
Date:   Tue, 25 Jun 2019 22:46:38 +0800
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Dear Thomas,

On 2019/6/24 17:45, Thomas Gleixner wrote:
> Zhiqiang,
> 
> On Mon, 24 Jun 2019, Zhiqiang Liu wrote:
>>
>> Thanks again for your detailed advice.
>> As your said, the max_softirq_time_usecs setting without explaining the
>> relationship with CONFIG_HZ will give a false sense of controlability. And
>> the time accuracy of jiffies will result in a certain difference between the
>> max_softirq_time_usecs set value and the actual value, which is in one jiffies
>> range.
>>
>> I will add these infomation in the sysctl documentation and changelog in v2 patch.
> 
> Please make the sysctl milliseconds based. That's the closest approximation
> of useful units for this. This still has the same issues as explained
> before but it's not off by 3 orders of magitude anymore.
>

I have a doubt about _msecs_to_jiffies funcs, especially when input m is equal to 0.

For different HZ setttings, different _msecs_to_jiffies funcs will be chosen for
msecs_to_jiffies func. However, the performance of different _msecs_to_jiffies is
inconsistent with input m is equal to 0.
If HZ satisfies the condition: HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ), the return
value of _msecs_to_jiffies func with m=0 is different with different HZ setting.
------------------------------------
| HZ |	MSEC_PER_SEC / HZ | return |
------------------------------------
|1000|		1	  |   0	   |
|500 |		2	  |   1	   |
|200 |		5	  |   1	   |
|100 |		10	  |   1	   |
------------------------------------

Why only the return value of HZ=1000 is equal to 0 with m=0 ?

Codes are given as follows,
    #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
    static inline unsigned long _msecs_to_jiffies(const unsigned int m)
    {
            return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
    }
    #elif HZ > MSEC_PER_SEC && !(HZ % MSEC_PER_SEC)
    static inline unsigned long _msecs_to_jiffies(const unsigned int m)
    {
            if (m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
                    return MAX_JIFFY_OFFSET;
            return m * (HZ / MSEC_PER_SEC);
    }
    #else
    static inline unsigned long _msecs_to_jiffies(const unsigned int m)
    {
            if (HZ > MSEC_PER_SEC && m > jiffies_to_msecs(MAX_JIFFY_OFFSET))
                    return MAX_JIFFY_OFFSET;

            return (MSEC_TO_HZ_MUL32 * m + MSEC_TO_HZ_ADJ32) >> MSEC_TO_HZ_SHR32;
    }


> Thanks,
> 
> 	tglx
>