Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a ring buffer

[zhangwei(Jovi)]

On Sat, Aug 10, 2013 at 9:26 AM, zhangwei(Jovi) <jovi.zhangwei@xxxxxxxxx> wrote:
> On Sat, Aug 10, 2013 at 12:20 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>>
>> Sorry, I didn't read this series yet. Not that I think this needs my
>> help, but I'll try to do this a later...
>>
>> On 08/09, Masami Hiramatsu wrote:
>> >
>> > I just concern using kmalloc() in the event handler.
>>
>> GFP_KERNEL should be fine for uprobe handler.
>>
>> However, iirc this conflicts with the patches from Jovi,
>> "Support ftrace_event_file base multibuffer" adds rcu_read_lock()
>> around uprobe_trace_print().
>
> (Sorry about html text rejected by kernel.org, send again with plain text.)
>
> Then we might need to call kmalloc before rcu_read_lock, also call kfree
> after rcu_read_unlock.
>
> And it's not needed to call kmalloc for each instances in multi-buffer
> case, just
> kmalloc once is enough.
>
> I also have same concern about use kmalloc in uprobe handler, use kmalloc
> in uprobe handler seems have a little overhead, why not pre-allocate one page
> static memory for temp buffer(perhaps trace_uprobe based)? one page size
> would be enough for all uprobe args storage, then we don't need to call
> kmalloc in that "fast path".
>
forgotten to say, that pre-allocated buffer would need to be per-cpu, to prevent
buffer corruption.

It's a memory space vs. performance trade-off problem. :)

>
> Thanks.
>>
>>
>> Steven, Jovi, what should we do with that patch? It seems that it
>> was forgotten.
>>
>> I can take these patches into my ubprobes branch and then ask Ingo
>> to pull. But this will complicate the routing of the new changes
>> like this.
>>
>> Oleg.
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