linux-kernel - Re: [patch] block: make struct block_device cacheline

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Message-ID: <x494nmu828c.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 19 Sep 2012 11:24:35 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	jaxboe@...ionio.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch] block: make struct block_device cacheline_aligned

Mikulas Patocka <mpatocka@...hat.com> writes:

> On Wed, 19 Sep 2012, Jeff Moyer wrote:
>
>> Jeff Moyer <jmoyer@...hat.com> writes:
>> 
>> > Hi,
>> >
>> > When testing against a pcie ssd or a ramdisk, making the block device
>> > structure cacheline_aligned provided a significant increase in
>> > performance:
>> 
>> Self-NACK on this one.  This results in a ton of warnings:
>> 
>> include/linux/fs.h:727: warning: ???__section__??? attribute does not
>> apply to types
>> In file included from include/linux/debugfs.h:18,
>>                  from kernel/trace/trace_probe.h:28,
>>                  from kernel/trace/trace_kprobe.c:23:
>> include/linux/fs.h:727: warning: ???__section__??? attribute does not
>> apply to types
>> 
>> And that leaves me with the task of figuring out if/why this actually
>> helps.
>> 
>> Cheers,
>> Jeff
>
> Hi
>
> Use ____cacheline_aligned instead of __cacheline_aligned

struct block_device is allocated as part of the bdev_inode:

struct bdev_inode {
        struct block_device bdev;
        struct inode vfs_inode;
};

The bdev_inode is allocated from the bdev_cachep, which uses
SLAB_HWCACHE_ALIGN.  So, in theory, this should already be aligned.

-Jeff
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