On Fri, Sep 20, 2013 at 07:12:47AM +0200, Ingo Molnar wrote:
>
> * Josef Bacik <jbacik@xxxxxxxxxxxx> wrote:
>
> > We can starve out the transaction commit with a bunch of caching threads
> > all running at the same time. This is because we will only drop the
> > extent_commit_sem if we need_resched(), which isn't likely to happen
> > since we will be reading a lot from the disk so have already
> > schedule()'ed plenty. Alex observed that he could starve out a
> > transaction commit for up to a minute with 32 caching threads all
> > running at once. This will allow us to drop the extent_commit_sem to
> > allow the transaction commit to swap the commit_root out and then all
> > the cachers will start back up. Thanks,
> >
> > Signed-off-by: Josef Bacik <jbacik@xxxxxxxxxxxx>
> > ---
> > fs/btrfs/extent-tree.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> > index cfb3cf7..cc074c34 100644
> > --- a/fs/btrfs/extent-tree.c
> > +++ b/fs/btrfs/extent-tree.c
> > @@ -442,7 +442,8 @@ next:
> > if (ret)
> > break;
> >
> > - if (need_resched()) {
> > + if (need_resched() ||
> > + rwsem_is_contended(&fs_info->extent_commit_sem)) {
> > caching_ctl->progress = last;
> > btrfs_release_path(path);
> > up_read(&fs_info->extent_commit_sem);
>
> So, just to fill in what happens in this loop:
>
> mutex_unlock(&caching_ctl->mutex);
> cond_resched();
> goto again;
>
> where 'again:' takes caching_ctl->mutex and fs_info->extent_commit_sem
> again:
>
> again:
> mutex_lock(&caching_ctl->mutex);
> /* need to make sure the commit_root doesn't disappear */
> down_read(&fs_info->extent_commit_sem);
>
> So, if I'm reading the code correct, there can be a fair amount of
> concurrency here: there may be multiple 'caching kthreads' per filesystem
> active, while there's one fs_info->extent_commit_sem per filesystem
> AFAICS.
>
> So, what happens if there are a lot of CPUs all busy holding the
> ->extent_commit_sem rwsem read-locked and a writer arrives? They'd all
> rush to try to release the fs_info->extent_commit_sem, and they'd block in
> the down_read() because there's a writer waiting.
>
> So there's a guarantee of forward progress. This should answer akpm's
> concern I think.
>
> If this analysis is correct then:
>
> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
>
Yup this is correct, thank you, I'll add your ack'ed by to the next iteration.
Josef
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