linux-kernel - Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF

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Date:	Thu, 31 Jul 2014 19:17:14 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ

On Thu, Jul 31, 2014 at 06:54:23PM +0200, Oleg Nesterov wrote:
> On 07/31, Frederic Weisbecker wrote:
> >
> > On Thu, Jul 31, 2014 at 06:42:46PM +0200, Oleg Nesterov wrote:
> > > On 07/31, Frederic Weisbecker wrote:
> > > >
> > > > On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote:
> > > > >
> > > > > At the end of the day, the syscall slowpath code calls a bunch of
> > > > > functions depending on what TIF_XYZ flags are set.  As long as it's
> > > > > structured like "if (TIF_A) do_a(); if (TIF_B) do_b();" or something
> > > > > like that, it's comprehensible.  But once random functions with no
> > > > > explicit flag checks or comments start showing up, it gets confusing.
> > > >
> > > > Yeah that's a point. I don't mind much the TIF_NOHZ test if you like.
> > >
> > > And in my opinion
> > >
> > > 	if (work & TIF_XYZ)
> > > 		user_exit();
> > >
> > > looks even more confusing. Because, once again, TIF_XYZ is not the
> > > reason to call user_exit().
> > >
> > > Not to mention this adds a minor performance penalty.
> >
> > That's a point too! You guys both convinced me! ;-)
> 
> Very nice, now I know that you can agree with 2 opposite opinions at
> the same time ;)

That's what an Acked-by from Schroedinger would look like!

> 
> > > > > If it's indeed all-or-nothing, I could remove the check and add a
> > > > > comment.  But please keep in mind that, currently, the slow path is
> > > > > *slow*, and my patches only improve the entry case.  So enabling
> > > > > context tracking on every task will hurt.
> > > >
> > > > That's what we do anyway. I haven't found a safe way to enabled context tracking
> > > > without tracking all CPUs.
> > >
> > > And if we change this, then the code above becomes racy. The state of
> > > TIF_XYZ can be changed right after the check. OK, it is racy anyway ;)
> > > but still this adds more confusion.
> >
> > No because all running tasks have this flag set when context tracking is
> > enabled. And context tracking can't be disabled on runtime.
> 
> Yes, yes, please note that I said "if we change this".

Yeah but the NO_HZ test wouldn't change much the situation I think.
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