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    SubjectRe: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count
    On Tue, May 25, 2010 at 04:49:12PM +0300, Artem Bityutskiy wrote:
    > From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
    >
    > The 'sync_supers' thread wakes up every 5 seconds (by default) and
    > writes back all super blocks. It keeps waking up even if there
    > are no dirty super-blocks. For many file-systems the superblock
    > becomes dirty very rarely, if ever, so 'sync_supers' does not do
    > anything most of the time.
    >
    > This patch improves 'sync_supers' and makes sleep if all superblocks
    > are clean and there is nothing to do. This helps saving the power.
    > This optimization is important for small battery-powered devices.

    > +void mark_sb_dirty(struct super_block *sb)
    > +{
    > + sb->s_dirty = 1;
    > +
    > + spin_lock(&supers_timer_lock);
    > + if (!supers_timer_armed) {
    > + bdi_arm_supers_timer();
    > + supers_timer_armed = 1;
    > + } else if (supers_timer_armed == -1)
    > + supers_timer_armed = 1;
    > + spin_unlock(&supers_timer_lock);
    > +}
    > +EXPORT_SYMBOL(mark_sb_dirty);

    Ouch... That turns a previously trivial operation into something
    much heavier; moreover, I'd rather see serious review of s_dirt
    uses.

    Note, e.g., that in your series you've touched udf; it can set s_dirt
    until the cows come home, but without ->write_super() it'll be ignored
    by everything in VFS and fs/udf itself never looks at the damn thing.

    A look around it shows fs/sysv, where we never clean the damn flag anymore
    for r/w mounts. Yes, really (got broken a year ago, nobody noticed).

    Or, e.g., BFS - there we have ->write_super() mark the buffer_head that
    contains on-disk sb dirty, and the only place that sets ->s_dirt is doing
    that immediately after having marked the same bh dirty itself. Interesting
    place, at that - bfs_fill_super() at r/w mount time... Note that ->sync_fs()
    there does *not* wait for anything, which is not the right thing to do.

    IOW, this thing is a good topic for code review; I suspect that quite a few
    users might be gone as the result.


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