Subject: Re: [Lse-tech] Re: [Patch 2/7] Add sysctl for schedstats
From: chandra seetharaman <sekharan@us.ibm.com>
Reply-To: sekharan@us.ibm.com
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Shailabh Nagar <nagar@watson.ibm.com>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       lse-tech <lse-tech@lists.sourceforge.net>
In-Reply-To: <44039860.8090708@yahoo.com.au>
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Organization: ibm
Date: Mon, 27 Feb 2006 17:42:37 -0800
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On Tue, 2006-02-28 at 11:25 +1100, Nick Piggin wrote:
> Chandra Seetharaman wrote:
> 
> >On Mon, 2006-02-27 at 20:17 +1100, Nick Piggin wrote:
> >
> >>> #ifdef CONFIG_SCHEDSTATS
> >>>+
> >>>+int schedstats_sysctl = 0;		/* schedstats turned off by default */
> >>>
> >>Should be read mostly.
> >>
> >>
> >>>+static DEFINE_PER_CPU(int, schedstats) = 0;
> >>>+
> >>>
> >>When the above is in the read mostly section, you won't need this at all.
> >>
> >>You don't intend to switch the sysctl with great frequency, do you?
> >>
> >
> >No, it is not expected to switch often.
> >
> >We originally coded it as __read_mostly, but thought the variable
> >bouncing between CPUs would be costly. Is it cheaper with
> >__read_mostly ? or it doesn't matter ?
> >
> >
> 
> Well it will only "bounce" when the cacheline it is in is written to by
> a different CPU. Considering this happens with your per-cpu implementation
> _anyway_, they don't buy you anything much.
> 
> Putting it in __read_mostly means that you won't happen to share a cacheline
> with a variable that is being written to frequently.
> 
Thanks for the clarification Nick.

> Nick
> 
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