linux-kernel - Re: per-cpu operation madness vs validation

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Date:	Wed, 27 Jul 2011 18:29:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: per-cpu operation madness vs validation

On Wed, 2011-07-27 at 18:20 +0200, Peter Zijlstra wrote:
> > > get_cpu_var()/put_cpu_var() were supposed to provide such delineation as
> > > well, but you've been actively destroying things like that with the
> > > recent per-cpu work.
> > 
> > The per cpu work is *not* focused on sections that access per cpu data, so
> > how could it destroy that? Nothing is changed there so far. The this_cpu
> > ops are introducing per cpu atomic operations that are safe and cheap
> > regardless of the execution context. The primary initial motivation was
> > incrementing per cpu counters without having to disabling interrupts
> > and/or preemption and it grew from there.
> 
> I think you need to look at 20b876918c065818b3574a426d418f68b4f8ad19 and
> try again. You removed get_cpu_var()/put_cpu_var() and replaced it with
> naked preempt_disable()/preempt_enable(). That's loosing information
> right there. 

Also things like the below hunk are just plain ugly and obfuscate the
code to safe one load at best. I'm sorely tempted to revert such crap.

@@ -1468,14 +1465,12 @@ static void x86_pmu_start_txn(struct pmu *pmu)
  */
 static void x86_pmu_cancel_txn(struct pmu *pmu)
 {
-       struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
-
-       cpuc->group_flag &= ~PERF_EVENT_TXN;
+       __this_cpu_and(cpu_hw_events.group_flag, ~PERF_EVENT_TXN);
        /*
         * Truncate the collected events.
         */
-       cpuc->n_added -= cpuc->n_txn;
-       cpuc->n_events -= cpuc->n_txn;
+       __this_cpu_sub(cpu_hw_events.n_added, __this_cpu_read(cpu_hw_events.n_txn));
+       __this_cpu_sub(cpu_hw_events.n_events, __this_cpu_read(cpu_hw_events.n_txn));
        perf_pmu_enable(pmu);
 }

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