linux-kernel - Re: [patch 02/24] perfmon: base code

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Date:	Thu, 27 Nov 2008 19:54:33 +0100
From:	"stephane eranian" <eranian@...glemail.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	"Andi Kleen" <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...e.hu, x86@...nel.org,
	sfr@...b.auug.org.au
Subject: Re: [patch 02/24] perfmon: base code

Thomas,

On Thu, Nov 27, 2008 at 7:36 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Thu, 27 Nov 2008, Andi Kleen wrote:
>
>> > Well, where is it checked ? Where is checked whether Oprofile runs or not ?
>>
>> That is done using the perfctr reservation. I saw that somewhere in the
>> patchkit. The NMI watchdog uses that too.
>>
>> > > The Oprofile reservation you see is built on top of the cpumask reservation.
>> > > It tries to allocate in one call and atomically ALL the CPUs as this is the way
>> > > Oprofile operates. Thus it fails if one perfmon system-wide session or one
>> > > perfmon per-thread exists.
>> >
>> > This only prevents oprofile from starting, but it does neither prevent
>> > thread sessions nor does it prevent a perfmon per cpu session on a cpu
>> > which was onlined after oprofile started, simply because it's bit is
>> > missing in the CPU mask.
>>
>> The perfctr reservation is global over all CPUs.
>
> So this mean we manage resources on multiple levels, some bits here
> and some bits there and five checks in each code path do get them all.
>
> Really convincing concept.

That is a different level of resource management: register level.

Both Perfmon and Oprofile can operate with fewer registers than what the
hardware actually supports. Take an AMD64 CPU with 4 counters. With
the NMI watchdog active, Perfmon or Oprofile will run with only 3. That's
better than nothing and is sufficient for many measurments.
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