linux-kernel - Re: [PATCH 1/2] Make the batch size of the percpu

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Date:	Tue, 30 Apr 2013 10:48:14 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] Make the batch size of the percpu_counter
 configurable

On Tue, 2013-04-30 at 17:28 +0000, Christoph Lameter wrote:
> On Tue, 30 Apr 2013, Tim Chen wrote:
> 
> > On Tue, 2013-04-30 at 13:32 +0000, Christoph Lameter wrote:
> > > On Mon, 29 Apr 2013, Tim Chen wrote:
> > >
> > > > diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> > > > index d5dd465..5ca7df5 100644
> > > > --- a/include/linux/percpu_counter.h
> > > > +++ b/include/linux/percpu_counter.h
> > > > @@ -22,6 +22,7 @@ struct percpu_counter {
> > > >  	struct list_head list;	/* All percpu_counters are on a list */
> > > >  #endif
> > > >  	s32 __percpu *counters;
> > > > +	int *batch ____cacheline_aligned_in_smp;
> > > >  };
> > >
> > > What is this for and why does it have that alignmend?
> >
> > I was assuming that if batch is frequently referenced, it probably
> > should not share a cache line with the counters field.
> 
> And why is it a pointer?

A pointer because the default percpu_counter_batch value could change
later when cpus come online after we initialize per cpu counter and
percpu_counter_batch will get computed again in percpu_counter_startup.
Making it a pointer will make it unnecessary to come back and change the
batch sizes if we use static batch value and default batch size.

> 
> And the pointer is so frequently changed that it needs it own cache line?
> 

On second thought, your're right. It is unnecessary for *batch to have
its own cache line as the counters pointer and head_list above it will
not change frequently.  I'll remove the cache alignment.

Tim

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