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

[xinhui]

On 2016å06æ29æ 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@xxxxxxxxxxxxxxxxxx]
> > > Sent: Monday, June 20, 2016 4:29 PM
> > > To: Byungchul Park; peterz@xxxxxxxxxxxxx; mingo@xxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx; npiggin@xxxxxxx; walken@xxxxxxxxxx;
> > > ak@xxxxxxx; tglx@xxxxxxxxxxxxxxxxxxxxxxxxxxx
> > > Subject: Re: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace
> > >
> > >
> > > On 2016å06æ20æ 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 = {