Re: [PATCH] blk-cgroup: Optimize blkcg_rstat_flush()

[Waiman Long]

On 5/31/22 15:03, Waiman Long wrote:
> On 5/31/22 14:24, Tejun Heo wrote:
> > 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?
>
> It is probably not a sufficient solution, but it is simple. The 
> problem with keeping a list of recently updated blkg's is that 
> sequence lock does not provide enough synchronization on the read side 
> to guarantee a race free reset of the list. It may be doable, but I 
> need to think harder on the best way to do it without too much overhead.
>
> Thanks,
> Longman
I think we can use a lockless list to keep a list of recently updated 
blkg for each blkcg. There is an atomic operation overhead when an entry 
is added to the list though. I will send an updated patch later today or 
tomorrow.

Thanks,
Longman