On Mon 25-01-16 10:03:24, Mel Gorman wrote:
> In the generic read paths the kernel looks up a page in the page cache
> and if it's up to date, it is used. If not, the page lock is acquired to
> wait for IO to complete and then check the page. If multiple processes
> are waiting on IO, they all serialise against the lock and duplicate the
> checks. This is unnecessary.
>
> The page lock in itself does not give any guarantees to the callers about
> the page state as it can be immediately truncated or reclaimed after the
> page is unlocked. It's sufficient to wait_on_page_locked and then continue
> if the page is up to date on wakeup.
>
> It is possible that a truncated but up-to-date page is returned but the
> reference taken during read prevents it disappearing underneath the caller
> and the data is still valid if PageUptodate.
>
> The overall impact is small as even if processes serialise on the lock,
> the lock section is tiny once the IO is complete. Profiles indicated that
> unlock_page and friends are generally a tiny portion of a read-intensive
> workload. An artifical test was created that had instances of dd access
> a cache-cold file on an ext4 filesystem and measure how long the read took.
>
> paralleldd
> 4.4.0 4.4.0
> vanilla avoidlock
> Amean Elapsd-1 5.28 ( 0.00%) 5.15 ( 2.50%)
> Amean Elapsd-4 5.29 ( 0.00%) 5.17 ( 2.12%)
> Amean Elapsd-7 5.28 ( 0.00%) 5.18 ( 1.78%)
> Amean Elapsd-12 5.20 ( 0.00%) 5.33 ( -2.50%)
> Amean Elapsd-21 5.14 ( 0.00%) 5.21 ( -1.41%)
> Amean Elapsd-30 5.30 ( 0.00%) 5.12 ( 3.38%)
> Amean Elapsd-48 5.78 ( 0.00%) 5.42 ( 6.21%)
> Amean Elapsd-79 6.78 ( 0.00%) 6.62 ( 2.46%)
> Amean Elapsd-110 9.09 ( 0.00%) 8.99 ( 1.15%)
> Amean Elapsd-128 10.60 ( 0.00%) 10.43 ( 1.66%)
>
> The impact is small but intuitively, it makes sense to avoid unnecessary
> calls to lock_page.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
The patch looks good. One small nit below, otherwise feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
> ---
> mm/filemap.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 49 insertions(+)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index aa38593d0cd5..235ee2b0b5da 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1649,6 +1649,15 @@ static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
> index, last_index - index);
> }
> if (!PageUptodate(page)) {
> + /*
> + * See comment in do_read_cache_page on why
> + * wait_on_page_locked is used to avoid unnecessarily
> + * serialisations and why it's safe.
> + */
> + wait_on_page_locked(page);
> + if (PageUptodate(page))
> + goto page_ok;
> +
We want a wait_on_page_locked_killable() here to match the
lock_page_killable() later in do_generic_file_read()?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>