linux-kernel - Re: [PATCH] Move mostly read variables to __read

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Message-ID: <4EA6645C.2010408@scalemp.com>
Date:	Tue, 25 Oct 2011 09:25:16 +0200
From:	Eial Czerwacki <eial@...lemp.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	linux-kernel@...r.kernel.org,
	"Shai Fultheim (Shai@...leMP.com)" <Shai@...lemp.com>
Subject: Re: [PATCH] Move mostly read variables to __read_mostly section.

On 10/24/2011 06:47 AM, Andi Kleen wrote:
> Eial Czerwacki<eial@...lemp.com>  writes:
>>    * Once the number of bh's in the machine exceeds this level, we start
>> --- a/kernel/time/timekeeping.c	2010-05-12 14:59:32.000000000 -0700
>> +++ b/kernel/time/timekeeping.c	2010-05-24 11:56:32.000000000 -0700
>> @@ -154,7 +154,7 @@ __cacheline_aligned_in_smp DEFINE_SEQLOC
>>    * used instead.
>>    */
>>   static struct timespec xtime __attribute__ ((aligned (16)));
>> -static struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
>> +static struct timespec wall_to_monotonic __attribute__ ((aligned
>>    (16))) __read_mostly;
>
> That means that a common time fetch/update call will usually have to fetch two
> cachelines now instead of one, because it needs xtime and
> wall_to_monotonic. Usually those end up nearby.
>
> On the other hand it's rare to fetch wall_to_monotonic without xtime.
>
> So if anything all those variables should be grouped together in a
> single cacheline, but definitely not split like you do.
>
> The other changes in your patch are fine for me.
>
> -Andi
Hello Andi,

Thanks for the input, we've removed the mentioned above section from the 
patch and it will be handled in a different patch.

the new patch is as follows:

Signed-off-by: Eial Czerwacki<eial@...leMP.com>
Signed-off-by: Shai Fultheim<Shai@...leMP.com>
---

--- a/fs/buffer.c	2010-05-12 14:59:32.000000000 -0700
+++ b/fs/buffer.c	2010-05-24 11:56:32.000000000 -0700
@@ -3233,7 +3233,7 @@ SYSCALL_DEFINE2(bdflush, int, func, long
  /*
   * Buffer-head allocation
   */
-static struct kmem_cache *bh_cachep;
+static struct kmem_cache __read_mostly *bh_cachep;

  /*
   * Once the number of bh's in the machine exceeds this level, we start


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