Re: [PATCH] debugobjects: add a new Kconfig for POOL_SIZE

[Waiman Long <longman@redhat.com>]

On 11/19/2018 10:17 AM, Qian Cai wrote:
> On Mon, 2018-11-19 at 09:51 -0500, Waiman Long wrote:
>> On 11/19/2018 08:27 AM, Qian Cai wrote:
>>> On 11/19/18 at 3:09 AM, Thomas Gleixner wrote:
>>>
>>>> Qian,
>>>>
>>>> On Sun, 18 Nov 2018, Qian Cai wrote:
>>>>>> On Nov 18, 2018, at 1:21 PM, Thomas Gleixner <tglx@linutronix.de>
>>>>>> wrote:
>>>>>> On Sun, 18 Nov 2018, Qian Cai wrote:
>>>>>>> As the results, systems have 60+ CPUs with both timer and workqueue
>>>>>>> objects enabled could trigger "ODEBUG: Out of memory. ODEBUG
>>>>>>> disabled".
>>>>>>>
>>>>>>> Hence, add a new Kconfig option so users could adjust
>>>>>>> ODEBUG_POOL_SIZE
>>>>>>> accordingly if either timer or workqueue objects are selected.
>>>>>> why do we need a config option, when the required number can be
>>>>>> deduced
>>>>>> already from the active CONFIG_DEBUG_OBJECTS_* and NR_CPUS?
>>>>>>
>>>>> It because I am worry about the coupling between the implementation
>>>>> details of
>>>>> timers and workqueue objects, and the computation in the code you
>>>>> mentioned
>>>>> here. For example, people could change workqueue.c to have different
>>>>> number
>>>>> of worekqueues initialized during the early boot in the future which is
>>>>> going to
>>>>> affect the required pool size, and I am not sure if people are going to
>>>>> adjust the
>>>>> code in debugobjects.c here as well when they made changes like that.
>>>>>
>>>>> Also, the computation could become so complex depends on lots of config
>>>>> options like perf, hrtimer, and combinations that I have not tested so
>>>>> far which is
>>>>> difficult to exhausted all the possibilities.
>>>>>
>>>>> Hence, I feel like the Kconfig option is more flexible and less error-
>>>>> prone.
>>>> Quite the contrary. Config options are a pain and truly error-prone if you
>>>> want to compile general purpose kernels as distributions do.
>>>>
>>> Ah, I never thought distributions people would
>>> enable those debugging options.
>> Distros like RHEL usually ship two kernels - one for production and one
>> for debug. The debug kernel does have debugobjects enabled.
>>
> Right, I can remember that now . However, if I understand correctly, since the
> early static pool size needs to be determined during the compilation time, it
> depends on the No. CPUs are from the machines that built the distro kernels.
> Then, when users use those distro kernels, they are not going to have correct
> the pool size according to the No. CPUs on their test machines.

I see your point. Perhaps you can make ODEBUG_POOL_SIZE scales with
CONFIG_NR_CPUS like

#define ODEBUG_POOL_SIZE    (1024 + CONFIG_NR_CPUS * 2)

CONFIG_NR_CPUS is usually set to a lot higher than the actual number of
CPUs in a typical system. So you don't want to set the multiplier too high.

Cheers,
Longman