linux-kernel - Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER

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Date:	Thu, 25 Feb 2016 09:40:57 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to
 entry_SYSENTER_32

* Mike Galbraith <umgwanakikbuti@...il.com> wrote:

> On Thu, 2016-02-25 at 09:14 +0100, Ingo Molnar wrote:
> 
> > But but ... 'context tracking' is not really something that a regular distro 
> > kernel cares about much - it's a nohz-full special AFAICS.

Let me qualify that: with the timer code maintenance hat on I really love all nohz 
variants (the deeper the better), but now I have my x86 maintainer hat on, and as 
such I'm really annoyed at those nohz folks adding overhead to the syscall hot 
path! ;-)

> (psst.. distros are shipping it)

Yeah, indeed, Fedora does - but AFAICS:

 fomalhaut:~> grep NO_HZ /boot/config-4.1.13-100.fc21.x86_64 
 CONFIG_NO_HZ_COMMON=y
 # CONFIG_NO_HZ_IDLE is not set
 CONFIG_NO_HZ_FULL=y
 # CONFIG_NO_HZ_FULL_ALL is not set
 # CONFIG_NO_HZ_FULL_SYSIDLE is not set
 CONFIG_NO_HZ=y
 CONFIG_RCU_FAST_NO_HZ=y

... which won't result in actual full-nohz CPUs unless you boot it with a special 
boot parameter, right?

What is the easiest way to query which/how many CPUs are in nohz-full mode and do 
context tracking? I somehow thought /proc/timer_* had that info, but that does not 
appear to be the case.

Thanks,

	Ingo