Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code

From: Frederic Weisbecker <fweisbec@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Rik van Riel <riel@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Brian Gerst <brgerst@gmail.com>,
	Denys Vlasenko <vda.linux@googlemail.com>,
	Kees Cook <keescook@chromium.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Oleg Nesterov <oleg@redhat.com>,
	Andrew Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-tip-commits@vger.kernel.org" 
	<linux-tip-commits@vger.kernel.org>
Subject: Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code
Date: Wed, 12 Aug 2015 15:13:04 +0200	[thread overview]
Message-ID: <20150812131302.GA21542@lerouge> (raw)
In-Reply-To: <CALCETrXHNwYT9b4RWWw3TSO_7LqX9csn=sE-Sa7oyy860BL0UQ@mail.gmail.com>

On Tue, Aug 11, 2015 at 03:59:37PM -0700, Andy Lutomirski wrote:
> On Tue, Aug 11, 2015 at 3:49 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > On Tue, Aug 11, 2015 at 03:25:04PM -0700, Andy Lutomirski wrote:
> >> Can you explain to me what context tracking does that rcu_irq_enter
> >> and vtime_account_irq_enter don't do that's expensive?  Frankly, I'd
> >> rather drop everything except the context tracking callback.
> >
> > Irqs have their own hooks in the generic code. irq_enter() and irq_exit().
> > And those take care of RCU and time accounting already. So arch code really
> > doesn't need to care about that.
> 
> I'd love to have irq_enter_from_user and irq_enter_from_kernel instead.

I don't get why we need that. Vtime internals already keeps track of where we
are. Again mixing up hard and soft tracking is asking for troubles.

> 
> >
> > context tracking exists for the sole purpose of tracking states that don't
> > have generic hooks. Those are syscalls and exceptions.
> >
> > Besides, rcu_user_exit() is more costly than rcu_irq_enter() which have been
> > designed for the very purpose of providing a fast RCU tracking for non sleepable
> > code (which needs rcu_user_exit()).
> >
> 
> So rcu_user_exit is slower because it's okay to sleep after calling it?
> 
> Would it be possible to defer the overhead until we actually try to
> sleep rather than doing it on entry?  (I have no idea what's going on
> under the hood.)

That's a question for Paul.

> Anyway, irq_enter_from_user would solve this problem completely.

How?

> >
> > I've been thinking about pushing down syscalls and exceptions to generic
> > handlers. It might work for syscalls btw. But many exceptions have only
> > arch handlers, or significant amount of work is done on the arch level
> > which might make use of RCU (eg: breakpoint handlers on x86).
> 
> I'm trying to port the meat of the x86 syscall code to C.  Maybe the
> result will generalize.  The exit code is already in C (in -tip).

But please don't change such semantics along the way, it really doesn't help
to review the x86 low level changes if it's mixed up with fundamental context
tracking changes.