From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH v6 3/3] blk-cgroup: Optimize blkcg_rstat_flush() Date: Wed, 8 Jun 2022 14:16:45 -0400 Message-ID: <506eaa3d-be84-c51e-3252-2979847054fe@redhat.com> References: <20220602192020.166940-1-longman@redhat.com> <20220602192020.166940-4-longman@redhat.com> <20220608165732.GB19399@blackbody.suse.cz> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1654712210; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=puhUcvLRx9zat7PF7KwdoaGeUx8bqAUgfHHvV+ON4tg=; b=frsC9osYGpWdpHxJoJ5yuwAHyOSL1r/4wq+GgW5t3STu+2R5jQ9RYs3q5FpVhCq3W+Rr4Z POgiK9t7KzttCoA1zMELCYWJqLG5nXE+mhhZ7BFcReAhMssRXAHhRdGLU/qs/GVUE5zhm8 bTqYdkUxTcJNJvuFJTF3zMEtwTs+kfE= Content-Language: en-US In-Reply-To: <20220608165732.GB19399@blackbody.suse.cz> List-ID: Content-Type: text/plain; charset="utf-8"; format="flowed" To: =?UTF-8?Q?Michal_Koutn=c3=bd?= Cc: Tejun Heo , Jens Axboe , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Ming Lei On 6/8/22 12:57, Michal Koutný wrote: > Hello. > > On Thu, Jun 02, 2022 at 03:20:20PM -0400, Waiman Long wrote: >> 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. > Yes, there's no point to flush stats for idle devices if there can be > many of them. Good idea. > >> +static struct llist_node *fetch_delete_blkcg_llist(struct llist_head *lhead) >> +{ >> + return xchg(&lhead->first, &llist_last); >> +} >> + >> +static struct llist_node *fetch_delete_lnode_next(struct llist_node *lnode) >> +{ >> + struct llist_node *next = READ_ONCE(lnode->next); >> + struct blkcg_gq *blkg = llist_entry(lnode, struct blkg_iostat_set, >> + lnode)->blkg; >> + >> + WRITE_ONCE(lnode->next, NULL); >> + percpu_ref_put(&blkg->refcnt); >> + return next; >> +} > Idea/just asking: would it make sense to generalize this into llist.c > (this is basically llist_del_first() + llist_del_all() with a sentinel)? > For the sake of reusability. I have thought about that. It can be done as a follow-up patch to add a sentinel version into llist and use that instead. Of course, I can also update this patchset to include that. > >> +#define blkcg_llist_for_each_entry_safe(pos, node, nxt) \ >> + for (; (node != &llist_last) && \ >> + (pos = llist_entry(node, struct blkg_iostat_set, lnode), \ >> + nxt = fetch_delete_lnode_next(node), true); \ >> + node = nxt) >> + > It's good hygiene to parenthesize the args. I am aware of that. I will certainly add that if it is a generic macro that can have many users. > >> @@ -2011,9 +2092,16 @@ 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); >> + } >> + > When a blkg's cgroup is rmdir'd, what happens with the lhead list? > We have cgroup_rstat_exit() in css_free_rwork_fn() that ultimately flushes rstats. > init_and_link_css however adds reference form blkcg->css to cgroup->css. > The blkcg->css would be (transitively) pinned by the lhead list and > hence would prevent the final flush (when refs drop to zero). Seems like > a cyclic dependency. > > Luckily, there's also per-subsys flushing in css_release which could be > moved after rmdir (offlining) but before last ref is gone: > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c > index adb820e98f24..d830e6a8fb3b 100644 > --- a/kernel/cgroup/cgroup.c > +++ b/kernel/cgroup/cgroup.c > @@ -5165,11 +5165,6 @@ static void css_release_work_fn(struct work_struct *work) > > if (ss) { > /* css release path */ > - if (!list_empty(&css->rstat_css_node)) { > - cgroup_rstat_flush(cgrp); > - list_del_rcu(&css->rstat_css_node); > - } > - > cgroup_idr_replace(&ss->css_idr, NULL, css->id); > if (ss->css_released) > ss->css_released(css); > @@ -5279,6 +5274,11 @@ static void offline_css(struct cgroup_subsys_state *css) > css->flags &= ~CSS_ONLINE; > RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL); > > + if (!list_empty(&css->rstat_css_node)) { > + cgroup_rstat_flush(css->cgrp); > + list_del_rcu(&css->rstat_css_node); > + } > + > wake_up_all(&css->cgroup->offline_waitq); > } > > (not tested) Good point. Your change may not be enough since there could be update after the flush which will pin the blkg and hence blkcg.  I guess one possible solution may be to abandon the llist and revert back to list iteration when offline. I need to think a bit more about that. > > >> u64_stats_update_end_irqrestore(&bis->sync, flags); >> if (cgroup_subsys_on_dfl(io_cgrp_subsys)) >> - cgroup_rstat_updated(bio->bi_blkg->blkcg->css.cgroup, cpu); >> + cgroup_rstat_updated(blkcg->css.cgroup, cpu); > Maybe bundle the lhead list maintenace with cgroup_rstat_updated() under > cgroup_subsys_on_dfl()? The stats can be read on v1 anyway. I don't quite understand here. The change is not specific to v1 or v2. What do you mean by the stat is readable on v1? Cheers, Longman