From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH -V3 01/11] percpu_counters: make fbc->count read atomic
 on 32 bit architecture
Date: Wed, 27 Aug 2008 21:09:25 -0700
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Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, cmm@us.ibm.com,
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To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
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On Thu, 28 Aug 2008 09:22:00 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> On Wed, Aug 27, 2008 at 02:22:50PM -0700, Andrew Morton wrote:
> > On Wed, 27 Aug 2008 23:01:52 +0200
> > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > 
> > > > 
> > > > > +static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> > > > > +{
> > > > > +	return fbc_count(fbc);
> > > > > +}
> > > > 
> > > > This change means that a percpu_counter_read() from interrupt context
> > > > on a 32-bit machine is now deadlockable, whereas it previously was not
> > > > deadlockable on either 32-bit or 64-bit.
> > > > 
> > > > This flows on to the lib/proportions.c, which uses
> > > > percpu_counter_read() and also does spin_lock_irqsave() internally,
> > > > indicating that it is (or was) designed to be used in IRQ contexts.
> > > 
> > > percpu_counter() never was irq safe, which is why the proportion stuff
> > > does all the irq disabling bits by hand.
> > 
> > percpu_counter_read() was irq-safe.  That changes here.  Needs careful
> > review, changelogging and, preferably, runtime checks.  But perhaps
> > they should be inside some CONFIG_thing which won't normally be done in
> > production.
> > 
> > otoh, percpu_counter_read() is in fact a rare operation, so a bit of
> > overhead probably won't matter.
> > 
> > (write-often, read-rarely is the whole point.  This patch's changelog's
> > assertion that "Since fbc->count is read more frequently and updated
> > rarely" is probably wrong.  Most percpu_counters will have their
> > fbc->count modified far more frequently than having it read from).
> 
> we may actually be doing percpu_counter_add. But that doesn't update
> fbc->count. Only if the local percpu values cross FBC_BATCH we update
> fbc->count. If we are modifying fbc->count more frequently than
> reading fbc->count then i guess we would be contenting of fbc->lock more.
> 
> 

Yep.  The frequency of modification of fbc->count is of the order of a
tenth or a hundredth of the frequency of
precpu_counter_<modification>() calls.

But in many cases the frequency of percpu_counter_read() calls is far
far less than this.  For example, the percpu_counter_read() may only
happen when userspace polls a /proc file.