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
if (list_empty(blkcg_wb_sleeper_list))
return false;
we don't need to be super accurate here. Thanks,
Josef
