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    SubjectRe: [PATCH v2] Make vm_max_readahead configurable at run-time
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    On Thursday 11 February 2010 10:43:41 Wu Fengguang wrote:
    > On Wed, Feb 10, 2010 at 09:52:40PM +0800, Nikanth Karthikesan wrote:
    > > On Wednesday 10 February 2010 16:35:51 Wu Fengguang wrote:
    > > > Nikanth,
    > > >
    > > > > Make vm_max_readahead configurable at run-time. Expose a sysctl knob
    > > > > in procfs to change it. This would ensure that new disks added would
    > > > > use this value as their default read_ahead_kb.
    > > >
    > > > Do you have use case, or customer demand for it?
    > >
    > > No body requested for it. But when doing some performance testing with
    > > readahead_kb re-compiling would be a pain, and thought that having a
    > > configurable default might be useful.
    >
    > I wonder why you need to recompile kernel in the tests.
    > There are three interfaces to change readahead size in runtime:
    >
    > blockdev --setra 1024 /dev/sda
    > echo 512 > /sys/block/*/queue/read_ahead_kb
    > echo 512 > /sys/devices/virtual/bdi/*/read_ahead_kb
    >

    Right, I did use that. But thought that a global tunable would be better. :-)

    > > > > Also filesystems which use default_backing_dev_info would also
    > > > > use this new value, even if they were already mounted.
    > > > >
    > > > > Currently xfs, btrfs, nilfs, raw, mtd use the
    > > > > default_backing_dev_info.
    > > >
    > > > This sounds like bad interface, in that users will be confused by the
    > > > tricky details of "works for new devices" and "works for some fs".
    > > >
    > > > One more tricky point is, btrfs/md/dm readahead size may not be
    > > > influenced if some of the component disks are hot added.
    > > >
    > > > So this patch is only going to work for hot-plugged disks that
    > > > contains _standalone_ filesystem. Is this typical use case in servers?
    > >
    > > Yes, it would work only if the top-level disk is hot-plugged/created.
    >
    > Or maybe what you really want is a kernel parameter for setting the
    > default readahead size at boot time?
    >
    > In another thread, Christian Ehrhardt recommended to add a config
    > option for it. If you like it, I can also do the kernel parameter
    > by the way.
    >

    Kernel parameter seems to be the right way to go. Does the attached patch look
    good?

    Thanks
    Nikanth

    From: Nikanth Karthikesan <knikanth@suse.de>

    Add new kernel parameter "readahead", which would be used
    as the value of VM_MAX_READAHEAD.

    Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>

    ---

    diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
    index 736d456..354e6f1 100644
    --- a/Documentation/kernel-parameters.txt
    +++ b/Documentation/kernel-parameters.txt
    @@ -2148,6 +2148,8 @@ and is between 256 and 4096 characters. It is defined in the file
    Format: <reboot_mode>[,<reboot_mode2>[,...]]
    See arch/*/kernel/reboot.c or arch/*/kernel/process.c

    + readahead= Default readahead value for block devices.
    +
    relax_domain_level=
    [KNL, SMP] Set scheduler's default relax_domain_level.
    See Documentation/cgroups/cpusets.txt.
    diff --git a/block/blk-core.c b/block/blk-core.c
    index 718897e..02ed748 100644
    --- a/block/blk-core.c
    +++ b/block/blk-core.c
    @@ -499,7 +499,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id)
    q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
    q->backing_dev_info.unplug_io_data = q;
    q->backing_dev_info.ra_pages =
    - (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
    + (vm_max_readahead_kb * 1024) / PAGE_CACHE_SIZE;
    q->backing_dev_info.state = 0;
    q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
    q->backing_dev_info.name = "block";
    diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
    index 1a822ce..a593578 100644
    --- a/fs/fuse/inode.c
    +++ b/fs/fuse/inode.c
    @@ -870,7 +870,7 @@ static int fuse_bdi_init(struct fuse_conn *fc, struct super_block *sb)
    int err;

    fc->bdi.name = "fuse";
    - fc->bdi.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
    + fc->bdi.ra_pages = (vm_max_readahead_kb * 1024) / PAGE_CACHE_SIZE;
    fc->bdi.unplug_io_fn = default_unplug_io_fn;
    /* fuse does it's own writeback accounting */
    fc->bdi.capabilities = BDI_CAP_NO_ACCT_WB;
    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index 60c467b..17825d7 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -1188,9 +1188,11 @@ int write_one_page(struct page *page, int wait);
    void task_dirty_inc(struct task_struct *tsk);

    /* readahead.c */
    -#define VM_MAX_READAHEAD 128 /* kbytes */
    +#define DEFAULT_VM_MAX_READAHEAD 128 /* kbytes */
    #define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */

    +extern unsigned long vm_max_readahead_kb;
    +
    int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
    pgoff_t offset, unsigned long nr_to_read);

    diff --git a/init/main.c b/init/main.c
    index 4cb47a1..7d5230a 100644
    --- a/init/main.c
    +++ b/init/main.c
    @@ -70,6 +70,7 @@
    #include <linux/sfi.h>
    #include <linux/shmem_fs.h>
    #include <trace/boot.h>
    +#include <linux/backing-dev.h>

    #include <asm/io.h>
    #include <asm/bugs.h>
    @@ -249,6 +250,16 @@ static int __init loglevel(char *str)

    early_param("loglevel", loglevel);

    +static int __init readahead(char *str)
    +{
    + vm_max_readahead_kb = memparse(str, &str) / 1024ULL;
    + default_backing_dev_info.ra_pages = vm_max_readahead_kb
    + * 1024 / PAGE_CACHE_SIZE;
    + return 0;
    +}
    +
    +early_param("readahead", readahead);
    +
    /*
    * Unknown boot options get handed to init, unless they look like
    * unused parameters (modprobe will find them in /proc/cmdline).
    diff --git a/mm/backing-dev.c b/mm/backing-dev.c
    index 0e8ca03..e33ff34 100644
    --- a/mm/backing-dev.c
    +++ b/mm/backing-dev.c
    @@ -18,7 +18,7 @@ EXPORT_SYMBOL(default_unplug_io_fn);

    struct backing_dev_info default_backing_dev_info = {
    .name = "default",
    - .ra_pages = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
    + .ra_pages = DEFAULT_VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
    .state = 0,
    .capabilities = BDI_CAP_MAP_COPY,
    .unplug_io_fn = default_unplug_io_fn,
    diff --git a/mm/readahead.c b/mm/readahead.c
    index 033bc13..516f8da 100644
    --- a/mm/readahead.c
    +++ b/mm/readahead.c
    @@ -17,6 +17,8 @@
    #include <linux/pagevec.h>
    #include <linux/pagemap.h>

    +unsigned long vm_max_readahead_kb = DEFAULT_VM_MAX_READAHEAD;
    +
    /*
    * Initialise a struct file's readahead state. Assumes that the caller has
    * memset *ra to zero.

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