linux-kernel - Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat

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Date:   Thu, 2 Jun 2022 07:46:45 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat_flush()

On Thu, Jun 02, 2022 at 01:26:10PM -0400, Waiman Long wrote:
> 
> On 6/2/22 12:58, Tejun Heo wrote:
> > Hello,
> > 
> > On Thu, Jun 02, 2022 at 09:35:43AM -0400, Waiman Long wrote:
> > > @@ -2011,9 +2076,16 @@ void blk_cgroup_bio_start(struct bio *bio)
> > >   	}
> > >   	bis->cur.ios[rwd]++;
> > > +	if (!READ_ONCE(bis->lnode.next)) {
> > > +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
> > > +
> > > +		llist_add(&bis->lnode, lhead);
> > > +		percpu_ref_get(&bis->blkg->refcnt);
> > Hmm... what guarantees that more than one threads race here? llist assumes
> > that there's a single writer for a given llist_node and the ref count would
> > be off too, right?
> 
> The llist_add() function is atomic. It calls into llist_add_batch() in
> lib/llist.c which uses cmpxchg() to make the change. There is a non-atomic
> version __llist_add() which may be problematic in this case. Note that irq
> is disabled in the u64_stats_update* critical section, there shouldn't be a
> racing thread running in the same cpu. Other cpus will modify their own
> version of lhead. Perhaps the non-atomic version can be used here as well.

Ah, right, this is per-cpu, so there can be no second writer trying to add
the same node at the same time. Can you add a comment explaining the overall
design / behavior? Other than that, please feel free to add

 Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun