linux-kernel - Re: x86's nmi_hz wrt. oprofile's nmi_timer

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 02 Feb 2009 15:14:39 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	mingo@...e.hu
Cc:	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	hpa@...or.com
Subject: Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c

From: David Miller <davem@...emloft.net>
Date: Fri, 30 Jan 2009 13:54:09 -0800 (PST)

> From: Ingo Molnar <mingo@...e.hu>
> Date: Fri, 30 Jan 2009 16:01:25 +0100
> 
> > 
> > * David Miller <davem@...emloft.net> wrote:
> > 
> > Reducing it to 1 HZ was kind of a performance hack: running NMIs at HZ 
> > needlessly interrupts the CPU HZ times a second. It's more than enough to 
> > have 1 nmi-watchdog tick per second to notice deadlocks that take longer 
> > than 5 seconds.
> 
> For the NMI watchdog's purposes I understand the intent, and this
> is perfectly fine.
> 
> The problem is that it stays at '1' when oprofile starts using the NMI
> watchdog, and we certainly want more than one oprofile tick per second
> :-)

Just making sure you understand the problem, here is the
sequence of events:

1) At bootup, the NMI watchdog is tested.

   It is tested with nmi_hz=HZ

2) If the test passes, nmi_hz is reduced down to '1'

As I stated, everything up to this point is fine.  Next:

3) oprofile initializes and if we choose to use the NMI
   timer for oprofile profiling it is implemented using
   a simple DIE_NMI notifier.

   However, nmi_hz is still just '1' which means that oprofile
   will only receive one sample per-second.  And this is definitely
   not what we want.

Somehow the code in arch/x86/oprofile/nmi_timer_int.c needs to
have an interface into the NMI watchdog core so that it can
increase nmi_hz back up to "HZ" when the NMI timer profiling is
enabled and back down to "1" when such profiling stops.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/