On Thu, Mar 07, 2019 at 05:10:53PM -0500, Josef Bacik wrote:
> On Thu, Mar 07, 2019 at 07:08:32PM +0100, Andrea Righi wrote:
> > Prevent priority inversion problem when a high-priority blkcg issues a
> > sync() and it is forced to wait the completion of all the writeback I/O
> > generated by any other low-priority blkcg, causing massive latencies to
> > processes that shouldn't be I/O-throttled at all.
> >
> > The idea is to save a list of blkcg's that are waiting for writeback:
> > every time a sync() is executed the current blkcg is added to the list.
> >
> > Then, when I/O is throttled, if there's a blkcg waiting for writeback
> > different than the current blkcg, no throttling is applied (we can
> > probably refine this logic later, i.e., a better policy could be to
> > adjust the throttling I/O rate using the blkcg with the highest speed
> > from the list of waiters - priority inheritance, kinda).
> >
> > Signed-off-by: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
> > ---
> > block/blk-cgroup.c | 131 +++++++++++++++++++++++++++++++
> > block/blk-throttle.c | 11 ++-
> > fs/fs-writeback.c | 5 ++
> > fs/sync.c | 8 +-
> > include/linux/backing-dev-defs.h | 2 +
> > include/linux/blk-cgroup.h | 23 ++++++
> > mm/backing-dev.c | 2 +
> > 7 files changed, 178 insertions(+), 4 deletions(-)
> >
> > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> > index 2bed5725aa03..4305e78d1bb2 100644
> > --- a/block/blk-cgroup.c
> > +++ b/block/blk-cgroup.c
> > @@ -1351,6 +1351,137 @@ struct cgroup_subsys io_cgrp_subsys = {
> > };
> > EXPORT_SYMBOL_GPL(io_cgrp_subsys);
> >
> > +#ifdef CONFIG_CGROUP_WRITEBACK
> > +struct blkcg_wb_sleeper {
> > + struct backing_dev_info *bdi;
> > + struct blkcg *blkcg;
> > + refcount_t refcnt;
> > + struct list_head node;
> > +};
> > +
> > +static DEFINE_SPINLOCK(blkcg_wb_sleeper_lock);
> > +static LIST_HEAD(blkcg_wb_sleeper_list);
> > +
> > +static struct blkcg_wb_sleeper *
> > +blkcg_wb_sleeper_find(struct blkcg *blkcg, struct backing_dev_info *bdi)
> > +{
> > + struct blkcg_wb_sleeper *bws;
> > +
> > + list_for_each_entry(bws, &blkcg_wb_sleeper_list, node)
> > + if (bws->blkcg == blkcg && bws->bdi == bdi)
> > + return bws;
> > + return NULL;
> > +}
> > +
> > +static void blkcg_wb_sleeper_add(struct blkcg_wb_sleeper *bws)
> > +{
> > + list_add(&bws->node, &blkcg_wb_sleeper_list);
> > +}
> > +
> > +static void blkcg_wb_sleeper_del(struct blkcg_wb_sleeper *bws)
> > +{
> > + list_del_init(&bws->node);
> > +}
> > +
> > +/**
> > + * blkcg_wb_waiters_on_bdi - check for writeback waiters on a block device
> > + * @blkcg: current blkcg cgroup
> > + * @bdi: block device to check
> > + *
> > + * Return true if any other blkcg different than the current one is waiting for
> > + * writeback on the target block device, false otherwise.
> > + */
> > +bool blkcg_wb_waiters_on_bdi(struct blkcg *blkcg, struct backing_dev_info *bdi)
> > +{
> > + struct blkcg_wb_sleeper *bws;
> > + bool ret = false;
> > +
> > + spin_lock(&blkcg_wb_sleeper_lock);
> > + list_for_each_entry(bws, &blkcg_wb_sleeper_list, node)
> > + if (bws->bdi == bdi && bws->blkcg != blkcg) {
> > + ret = true;
> > + break;
> > + }
> > + spin_unlock(&blkcg_wb_sleeper_lock);
> > +
> > + return ret;
> > +}
>
> No global lock please, add something to the bdi I think? Also have a fast path
> of
OK, I'll add a list per-bdi and a lock as well.
>
> if (list_empty(blkcg_wb_sleeper_list))
> return false;
OK.
>
> we don't need to be super accurate here. Thanks,
>
> Josef
Thanks,
-Andrea
