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SubjectRe: Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a ring buffer
(2013/08/27 17:07), Namhyung Kim wrote:
> Hi Steven,
>
> On Thu, 22 Aug 2013 21:08:30 -0400, Steven Rostedt wrote:
>> On Fri, 23 Aug 2013 07:57:15 +0800
>> "zhangwei(Jovi)" <jovi.zhangwei@huawei.com> wrote:
>>
>>
>>>>
>>>> What about creating a per cpu buffer when uprobes are registered, and
>>>> delete them when they are finished? Basically what trace_printk() does
>>>> if it detects that there are users of trace_printk() in the kernel.
>>>> Note, it does not deallocate them when finished, as it is never
>>>> finished until reboot ;-)
>>>>
>>>> -- Steve
>>>>
>>> I also thought out this approach, but the issue is we cannot fetch user
>>> memory into per-cpu buffer, because use per-cpu buffer should under
>>> preempt disabled, and fetching user memory could sleep.
>>
>> Actually, we could create a per_cpu mutex to match the per_cpu buffers.
>> This is not unlike what we do in -rt.
>>
>> int cpu;
>> struct mutex *mutex;
>> void *buf;
>>
>>
>> /*
>> * Use per cpu buffers for fastest access, but we might migrate
>> * So the mutex makes sure we have sole access to it.
>> */
>>
>> cpu = raw_smp_processor_id();
>> mutex = per_cpu(uprobe_cpu_mutex, cpu);
>> buf = per_cpu(uprobe_cpu_buffer, cpu);
>>
>> mutex_lock(mutex);
>> store_trace_args(..., buf,...);
>> mutex_unlock(mutex);
>>
>
> Great! I'll go with this approach. Is it OK to you, Masami?

Yeah, it also seems to work. Please feel free to try it :)

Thank you,

--
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com




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