On Tue, Apr 24, 2012 at 09:11:15PM +0300, Sergei Trofimovich wrote:
> On Tue, 24 Apr 2012 13:32:37 -0400
> Josef Bacik <jo...@redhat.com> wrote:
>
> > On Tue, Apr 24, 2012 at 08:26:44PM +0300, Sergei Trofimovich wrote:
> > > From: Sergei Trofimovich <sly...@gentoo.org>
> > >
> > > Changing 'mount -oremount,thread_pool=2 /' didn't make any effect:
> > >
> > > maximum amount of worker threads is specified in 2 places:
> > > - in 'strict btrfs_fs_info::thread_pool_size'
> > > - in each worker struct: 'struct btrfs_workers::max_workers'
> > >
> > > 'mount -oremount' updated only 'btrfs_fs_info::thread_pool_size'.
> > >
> > > Fix it by pushing new maximum value to all created worker structures
> > > as well.
> > >
> > > Cc: Josef Bacik <jo...@redhat.com>
> > > Cc: Chris Mason <chris.ma...@oracle.com>
> > > Signed-off-by: Sergei Trofimovich <sly...@gentoo.org>
> > > ---
> > > WARNING: This patch makes sense only with
> > > WARNING: "btrfs: fix early abort in 'remount'"
> > > WARNING: sitting in Josef's -next branch.
> > > fs/btrfs/super.c | 38 +++++++++++++++++++++++++++++++++-----
> > > 1 files changed, 33 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> > > index 2f28fc0..ed2dab9 100644
> > > --- a/fs/btrfs/super.c
> > > +++ b/fs/btrfs/super.c
> > > @@ -435,11 +435,8 @@ int btrfs_parse_options(struct btrfs_root *root,
> > > char *options)
> > > case Opt_thread_pool:
> > > intarg = 0;
> > > match_int(&args[0], &intarg);
> > > - if (intarg) {
> > > + if (intarg)
> > > info->thread_pool_size = intarg;
> > > - printk(KERN_INFO "btrfs: thread pool %d\n",
> > > - info->thread_pool_size);
> > > - }
> > > break;
> > > case Opt_max_inline:
> > > num = match_strdup(&args[0]);
> > > @@ -1119,6 +1116,35 @@ error_fs_info:
> > > return ERR_PTR(error);
> > > }
> > >
> > > +static void btrfs_set_max_workers(struct btrfs_workers *workers, int
> > > new_limit)
> > > +{
> > > + workers->max_workers = new_limit;
> > > +}
> >
> > This needs to be protected by it's spin lock, so do
> >
> > spin_lock_irq(&workers->lock);
> > workers->max_workers = new_limit;
> > spin_unlock_irq(&workers->lock);
> >
> > Also this doesn't do anything for thread pools that have already gone above
> > the
> > new maximum. In practice this shouldn't happen really since most of these
> > workers are related to writing anyway, but it may be prudent in the future
> > to
> > make btrfs_set_max_workers to go through and stop threads until the workers
> > are
> > below the new limit. This doesn't have to be done in this patch, just good
> > idea
> > for future work.
>
> I've found a place where value is seemingly read w/o lock:
> /* fs/btrfs/disk-io.c */
> unsigned long btrfs_async_submit_limit(struct btrfs_fs_info *info)
> {
> unsigned long limit = min_t(unsigned long,
> info->workers.max_workers,
> info->fs_devices->open_devices);
> return 256 * limit;
> }
>
> How hot that path is? Is it fine to guard with similar lock
> or it's better to consider something more SMP friendly?
It's pretty hot, we're ok reading from the value in this case without a lock
since we just want to try and throttle ourselves, it's not important to be super
accurate. But in your case you are actually setting a new value which is going
to affect how we start new threads so you definitely want to lock it. Thanks,
Josef
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