Re: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace

[xinhui <xinhui.pan@linux.vnet.ibm.com>]

On 2016年06月29日 20:43, Byungchul Park wrote:
> On Mon, Jun 20, 2016 at 04:50:37PM +0900, byungchul.park wrote:
>>> -----Original Message-----
>>> From: xinhui [mailto:xinhui.pan@linux.vnet.ibm.com]
>>> Sent: Monday, June 20, 2016 4:29 PM
>>> To: Byungchul Park; peterz@infradead.org; mingo@kernel.org
>>> Cc: linux-kernel@vger.kernel.org; npiggin@suse.de; walken@google.com;
>>> ak@suse.de; tglx@inhelltoy.tec.linutronix.de
>>> Subject: Re: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace
>>>
>>>
>>> On 2016年06月20日 12:55, Byungchul Park wrote:
>>>> Currently, x86 implementation of save_stack_trace() is walking all stack
>>>> region word by word regardless of what the trace->max_entries is.
>>>> However, it's unnecessary to walk after already fulfilling caller's
>>>> requirement, say, if trace->nr_entries >= trace->max_entries is true.
>>>>
>>>> For example, CONFIG_LOCKDEP_CROSSRELEASE implementation calls
>>>> save_stack_trace() with max_entries = 5 frequently. I measured its
>>>> overhead and printed its difference of sched_clock() with my QEMU x86
>>>> machine.
>>>>
>>>> The latency was improved over 70% when trace->max_entries = 5.
>>>>
>>> [snip]
>>>
>>>> +static int save_stack_end(void *data)
>>>> +{
>>>> +	struct stack_trace *trace = data;
>>>> +	return trace->nr_entries >= trace->max_entries;
>>>> +}
>>>> +
>>>>    static const struct stacktrace_ops save_stack_ops = {
>>>>    	.stack		= save_stack_stack,
>>>>    	.address	= save_stack_address,
>>> then why not check the return value of ->address(), -1 indicate there is
>>> no room to store any pointer.
>>
>> Hello,
>>
>> Indeed. It also looks good to me even though it has to propagate the condition
>> between callback functions. I will modify it if it's better.
>
> Do you also think it would be better to make it propagate the result of
> ->address() rather than add a new callback, say, end_walk?
>
It's up to you. In my opinion, end_walk is better for reading.
  
>>
>> Thank you.
>> Byungchul
>>
>>>
>>>>    	.walk_stack	= print_context_stack,
>>>> +	.end_walk	= save_stack_end,
>>>>    };
>>>>
>>>>    static const struct stacktrace_ops save_stack_ops_nosched = {
>>>>
>