linux-kernel - Re: [PATCH] RCU: remove alignment padding from rcu

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Message-ID: <20110508145323.GS2641@linux.vnet.ibm.com>
Date:	Sun, 8 May 2011 07:53:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Richard Kennedy <richard@....demon.co.uk>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH] RCU: remove alignment padding from rcu_data on 64 bit
 builds

On Fri, May 06, 2011 at 01:13:56PM +0100, Richard Kennedy wrote:
> On Fri, 2011-05-06 at 08:20 +0200, Eric Dumazet wrote:
> ...
> > We could shrink this structure more, converting some 64bit fields to
> > 32bit ones.
> > 
> > Rationale is the algo is working well on 32bit arches, no need to use
> > 64bit fields.
> > 
> > candidates : completed, gpnum, passed_quiesc_completed, qlen,
> > qlen_last_fqs_check, blimit.
> > 
> > Counters might be converted too.
> 
> I don't know the code well enough to make that decision ;)
> Is it possible to shrink it enough to free up another cache line ?
> (16 longs to ints).

A number of these could be changed from long to int, though appropriate
adjustments need to be made.  Some of the fields could be placed under
#ifdef CONFIG_RCU_TRACE, though again, corresponding adjustments would
need to be made.

> CONFIG_NO_HZ adds 24 bytes so even for users with !CONFIG_NO_HZ the
> variables you suggest are not enough to free a cache line.  

Yep.  And recent work introducing RCU priority boosting adds some more.

But it is not like this structure is allocated as a unit, so are you
really all that sensitive to the exact size?

							Thanx, Paul
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