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    SubjectRe: [PATCH 3/3] writeback: avoid extra sync work at enqueue time
    On Mon 02-05-11 11:17:53, Wu Fengguang wrote:
    > This removes writeback_control.wb_start and does more straightforward
    > sync livelock prevention by setting .older_than_this to prevent extra
    > inodes from being enqueued in the first place.
    >
    > --- linux-next.orig/fs/fs-writeback.c 2011-05-02 11:17:24.000000000 +0800
    > +++ linux-next/fs/fs-writeback.c 2011-05-02 11:17:27.000000000 +0800
    > @@ -683,10 +672,12 @@ static long wb_writeback(struct bdi_writ
    > * (quickly) tag currently dirty pages
    > * (maybe slowly) sync all tagged pages
    > */
    > - if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_sync)
    > + if (wbc.sync_mode == WB_SYNC_ALL || wbc.tagged_sync) {
    > write_chunk = LONG_MAX;
    > + oldest_jif = jiffies;
    > + wbc.older_than_this = &oldest_jif;
    > + }
    What are the implications of not doing dirty-time livelock avoidance for
    other types of writeback? Is that a mistake? I'd prefer to have in
    wb_writeback():
    if (wbc.for_kupdate)
    oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval * 10);
    else
    oldest_jif = jiffies;
    wbc.older_than_this = &oldest_jif;

    And when you have this, you can make wbc.older_than_this just a plain
    number and remove all those checks for wbc.older_than_this == NULL.

    Honza

    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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