linux-kernel - Re: [GIT PULL] oprofile fixes for v2.6.40

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Message-ID: <20110527122301.GA32754@elte.hu>
Date:	Fri, 27 May 2011 14:23:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Robert Richter <robert.richter@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"oprofile-list@...ts.sourceforge.net" 
	<oprofile-list@...ts.sourceforge.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [GIT PULL] oprofile fixes for v2.6.40

* Robert Richter <robert.richter@....com> wrote:

> Robert Richter (1):
>       oprofile, x86: Enable preemption during pci device setup in IBS init

I'm wondering about this bit, as from a preempt_enable()/disable() 
pattern POV it looks bogus. It does:

        preempt_disable();
        for (offset = 1; offset < APIC_EILVT_NR_MAX; offset++) {
                if (get_eilvt(offset))
                        break;
        }
        preempt_enable();

	...

	ret = setup_ibs_ctl(offset);

So whatever condition it established at the get_eilvt() stage might 
not be valid at the setup_ibs_ctl() point: we might be on a different 
CPU.

Or are the IBS MSRs (which get twiddled in get_eilvt() absolutely 
symmetric across all CPUs, thus making this a preempt invariant 
thing? In that case we'd at least need a big fat comment in the code 
as it's not obvious at all ...

But i really do not see how this can work. get_eilvt() twiddles the 
APIC LVT of this current CPU. Does IBS only activate on a single CPU? 
How will this LVT entry be cleared on deinit?

Mysteries, mysteries!

Thanks,

	Ingo
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