linux-kernel - Re: [PATCH v2 1/2] tmpfs: Quick token library to allow scalable retrieval of tokens from token jar

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Message-Id: <20100609154102.9795ff07.akpm@linux-foundation.org>
Date:	Wed, 9 Jun 2010 15:41:02 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2 1/2] tmpfs: Quick token library to allow scalable
 retrieval of tokens from token jar

On Wed, 02 Jun 2010 10:32:49 -0700
Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> On Tue, 2010-06-01 at 14:51 -0700, Andrew Morton wrote:
> 
> > >
> > > +static void qtoken_reap_cache(struct qtoken *token_jar)
> > > +{
> > > +	long cnt;
> > > +	int i;
> > > +
> > > +	for_each_possible_cpu(i) {
> > > +		atomic_long_t	*cache;
> > > +
> > > +		cache = per_cpu_ptr(token_jar->cache, i);
> > > +		cnt = atomic_long_xchg(cache, -1);
> > > +		if (cnt > 0)
> > > +			token_jar->pool += cnt;
> > > +	}
> > > +}
> > 
> > Not cpu-hotplug aware.  Also, inefficient if num_online_cpus is much
> > less than num_possible_cpus.
> > 
> > It's really not hard to do!  lib/percpu_counter.c is per-cpu aware and
> > it does this by sneakily connecting all counters together system-wide.
> > 
> 
> I've thought about using "for_each_online_cpu" in the above loop.
> However, it someone takes a cpu offline, then we will lose the tokens
> in that cpu's cache when we do reap cache.

No, that's what cpu hotplug notifiers are for.  pecpu_counters does all
this.

> > > +	/* We should have acquired lock of token pool before coming here */
> > > +	if (token_jar->pool < (reserve + tokens))
> > > +		qtoken_reap_cache(token_jar);
> > > +	if (token_jar->pool >= (reserve + tokens)) {
> > > +		token_jar->pool -= tokens;
> > > +		allocated = 0;
> > > +	}
> > > +	return allocated;
> > > +}
> > 
> > ENOSPC means "your disk is full".  But my disk isn't full.  If this
> > inappropriate (indeed, incorrect) error code is ever propagated back to
> > userspace then users will be justifiably confused.
> > 
> 
> Originally, I was using -ENOSPC to mean tmpfs being full and out of
> space.  Will -EBUSY to denote all resources are used be more
> appropriate?  
> 

Return the number of tokens which were allocated?
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