linux-kernel - Re: Perhaps a side effect regarding NMI returns

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Date:	Tue, 29 Nov 2011 17:51:26 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Turner <pjt@...gle.com>
Subject: Re: Perhaps a side effect regarding NMI returns

On Tue, 2011-11-29 at 17:14 -0500, Jason Baron wrote:
> On Tue, Nov 29, 2011 at 03:58:21PM -0500, Steven Rostedt wrote:

> > But people don't like the overhead that stop_machine() causes, and I
> > have code that can make the modifications for ftrace with break points.
> > By adding a break point, syncing, then modifying the code and break
> 
> But if there's still has to be some sort of 'syncing' after we add a break
> point, how much are we going to save? Or I guess your're using an IPI? 

Well, anything is better than stop machine, event synchronize_sched() ;)

But the code I have in ftrace does bulk changes. It adds a break point
to all functions, then it does the sync, then it updates all the points
to the new code.

Looking at my code, here's what I did after setting up the breakpoints:

static void do_sync_core(void *data)
{
        sync_core();
}

static void run_sync(void)
{
        int enable_irqs = irqs_disabled();

        /* We may be called with interrupts disbled. */
        if (enable_irqs)
                local_irq_enable();
        on_each_cpu(do_sync_core, NULL, 1);
        if (enable_irqs)
                local_irq_disable();
}

Note, it's fine to enable interrupts here, it's only used by ftrace.

-- Steve


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