Hello, Waiman. On Tue, May 31, 2022 at 02:18:21PM -0400, Waiman Long wrote: > For a system with many CPUs and block devices, the time to do > blkcg_rstat_flush() from cgroup_rstat_flush() can be rather long. It > can be especially problematic as interrupt is disabled during the flush. > It was reported that it might take seconds in some extreme cases leading > to hard lockup messages. > > As it is likely that not all the percpu blkg_iostat_set's has been > updated since the last flush, those stale blkg_iostat_set's don't need > to be flushed in this case. This patch optimizes blkcg_rstat_flush() > by checking the current sequence number against the one recorded since > the last flush and skip the blkg_iostat_set if the sequence number > hasn't changed. There is a slight chance that it may miss an update > that is being done in parallel, the new update will just have to wait > until the next flush. > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx> > --- > block/blk-cgroup.c | 18 +++++++++++++++--- > block/blk-cgroup.h | 1 + > 2 files changed, 16 insertions(+), 3 deletions(-) > > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c > index 40161a3f68d0..79b89af61ef2 100644 > --- a/block/blk-cgroup.c > +++ b/block/blk-cgroup.c > @@ -864,11 +864,23 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) > unsigned long flags; > unsigned int seq; > > + seq = u64_stats_fetch_begin(&bisc->sync); > + /* > + * If the sequence number hasn't been updated since the last > + * flush, we can skip this blkg_iostat_set though we may miss > + * an update that is happening in parallel. > + */ > + if (seq == bisc->last_seq) > + continue; Is this a sufficient solution? The code assumes that there aren't too many blkgs for the cgroup, which can be wrong in some cases. Wouldn't it be better to create a list of updated blkg's per blkcg so that we don't walk all the dormant ones? Thanks. -- tejun
