Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation

From: Nick Piggin <npiggin@suse.de>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: paulmck@linux.vnet.ibm.com,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Avi Kivity <avi@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
	Rik van Riel <riel@redhat.com>, Ingo Molnar <mingo@elte.hu>,
	akpm@linux-foundation.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	David Miller <davem@davemloft.net>,
	Hugh Dickins <hugh.dickins@tiscali.co.uk>,
	Mel Gorman <mel@csn.ul.ie>
Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation
Date: Fri, 16 Apr 2010 19:22:24 +1000	[thread overview]
Message-ID: <20100416092224.GL5683@laptop> (raw)
In-Reply-To: <1271406542.13059.188.camel@pasglop>

On Fri, Apr 16, 2010 at 06:29:02PM +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2010-04-16 at 18:18 +1000, Nick Piggin wrote:
> > On Fri, Apr 16, 2010 at 04:51:34PM +1000, Benjamin Herrenschmidt wrote:
> > > On Mon, 2010-04-12 at 20:43 -0700, Paul E. McKenney wrote:
> > > > > So we might have to support the interrupt assumption, at least in
> > > > some
> > > > > form, with those guys...
> > > > 
> > > > One way to make the interrupt assumption official is to use
> > > > synchronize_sched() rather than synchronize_rcu().
> > > 
> > > Ok, so I'm a bit of a RCU newbie as you may know :-) Right now, we use
> > > neither, we use call_rcu and we free the pages from the callback.
> > 
> > BTW. you currently have an interesting page table freeing path where
> > you usually free by RCU, but (occasionally) free by IPI. This means
> > you need to disable both RCU and interrupts to walk page tables.
> 
> Well, the point is we use interrupts to synchronize. The fact that RCU
> used to do the job was an added benefit. I may need to switch to rcu
> _sched variants tho to keep that. The IPI case is a slow path in case we
> are out of memory and cannot allocate our page of RCU batch.

It is the slowpath but it forces all lookup paths to do irq disable
too.

> > If you change it to always use RCU, then you wouldn't need to disable
> > interrupts. Whether this actually matters anywhere in your mm code, I
> > don't know (it's probably not terribly important for gup_fast). But
> > rcu disable is always preferable for latency and performance.
> 
> Well, the main case is the hash miss and that always runs with IRQs off.

Probably not a big deal then.