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Message-ID: <44039860.8090708@yahoo.com.au>
Date: Tue, 28 Feb 2006 11:25:04 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: sekharan@us.ibm.com
CC: nagar@watson.ibm.com, linux-kernel <linux-kernel@vger.kernel.org>,
       lse-tech <lse-tech@lists.sourceforge.net>
Subject: Re: [Lse-tech] Re: [Patch 2/7] Add sysctl for schedstats
References: <1141026996.5785.38.camel@elinux04.optonline.net>	 <1141027367.5785.42.camel@elinux04.optonline.net>	 <1141027923.5785.50.camel@elinux04.optonline.net>	 <4402C3BB.7010705@yahoo.com.au> <1141067382.4770.699.camel@linuxchandra>
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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.

Nick

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