linux-kernel - Re: [RFC 12/12] x86/dumpstack: Optimize save_stack

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Message-ID: <20160629124343.GQ2279@X58A-UD3R>
Date:	Wed, 29 Jun 2016 21:43:43 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	peterz@...radead.org, mingo@...nel.org
Cc:	linux-kernel@...r.kernel.org, npiggin@...e.de, walken@...gle.com,
	ak@...e.de, xinhui.pan@...ux.vnet.ibm.com
Subject: Re: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace

On Mon, Jun 20, 2016 at 04:50:37PM +0900, byungchul.park wrote:
> > -----Original Message-----
> > From: xinhui [mailto:xinhui.pan@...ux.vnet.ibm.com]
> > Sent: Monday, June 20, 2016 4:29 PM
> > To: Byungchul Park; peterz@...radead.org; mingo@...nel.org
> > Cc: linux-kernel@...r.kernel.org; npiggin@...e.de; walken@...gle.com;
> > ak@...e.de; tglx@...elltoy.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?

> 
> Thank you.
> Byungchul
> 
> > 
> > >   	.walk_stack	= print_context_stack,
> > > +	.end_walk	= save_stack_end,
> > >   };
> > >
> > >   static const struct stacktrace_ops save_stack_ops_nosched = {
> > >