Re: [PATCH 1/3] rcu: Reduce synchronize_rcu() waiting time

[Uladzislau Rezki]

On Wed, Oct 25, 2023 at 10:17:22AM -0700, Boqun Feng wrote:
> On Wed, Oct 25, 2023 at 04:09:13PM +0200, Uladzislau Rezki (Sony) wrote:
> > A call to a synchronize_rcu() can be optimized from time point of
> > view. Different workloads can be affected by this especially the
> > ones which use this API in its time critical sections.
> > 
> > For example if CONFIG_RCU_NOCB_CPU is set, the wakeme_after_rcu()
> > callback can be delayed and such delay depends on:
> > 
> > - where in a nocb list it is located;
> > - how fast previous callbacks completed.
> > 
> > 1. On our Android devices i can easily trigger the scenario when
> > it is a last in the list out of ~3600 callbacks:
> > 
> 
> I wonder how many of the callbacks are queued via call_rcu_hurry()? If
> not a lot, I wonder whether we can resolve the problem differently, see
> below:
> 
It might be many. call_rcu_hurry() is still a new API. I expect the
usage of it will be increased especially in places where the memory
have to be reclaimed asap, for example small devices which are suffering
from the OOM killer.

> > <snip>
> >   <...>-29      [001] d..1. 21950.145313: rcu_batch_start: rcu_preempt CBs=3613 bl=28
> > ...
> >   <...>-29      [001] ..... 21950.152578: rcu_invoke_callback: rcu_preempt rhp=00000000b2d6dee8 func=__free_vm_area_struct.cfi_jt
> >   <...>-29      [001] ..... 21950.152579: rcu_invoke_callback: rcu_preempt rhp=00000000a446f607 func=__free_vm_area_struct.cfi_jt
> >   <...>-29      [001] ..... 21950.152580: rcu_invoke_callback: rcu_preempt rhp=00000000a5cab03b func=__free_vm_area_struct.cfi_jt
> >   <...>-29      [001] ..... 21950.152581: rcu_invoke_callback: rcu_preempt rhp=0000000013b7e5ee func=__free_vm_area_struct.cfi_jt
> >   <...>-29      [001] ..... 21950.152582: rcu_invoke_callback: rcu_preempt rhp=000000000a8ca6f9 func=__free_vm_area_struct.cfi_jt
> >   <...>-29      [001] ..... 21950.152583: rcu_invoke_callback: rcu_preempt rhp=000000008f162ca8 func=wakeme_after_rcu.cfi_jt
> >   <...>-29      [001] d..1. 21950.152625: rcu_batch_end: rcu_preempt CBs-invoked=3612 idle=....
> > <snip>
> > 
> > 2. We use cpuset/cgroup to classify tasks and assign them into
> > different cgroups. For example "backgrond" group which binds tasks
> > only to little CPUs or "foreground" which makes use of all CPUs.
> > Tasks can be migrated between groups by a request if an acceleration
> > is needed.
> > 
> > See below an example how "surfaceflinger" task gets migrated.
> > Initially it is located in the "system-background" cgroup which
> > allows to run only on little cores. In order to speed it up it
> > can be temporary moved into "foreground" cgroup which allows
> > to use big/all CPUs:
> > 
> > cgroup_attach_task():
> >  -> cgroup_migrate_execute()
> >    -> cpuset_can_attach()
> >      -> percpu_down_write()
> >        -> rcu_sync_enter()
> >          -> synchronize_rcu()
> >    -> now move tasks to the new cgroup.
> >  -> cgroup_migrate_finish()
> > 
> > <snip>
> >          rcuop/1-29      [000] .....  7030.528570: rcu_invoke_callback: rcu_preempt rhp=00000000461605e0 func=wakeme_after_rcu.cfi_jt
> >     PERFD-SERVER-1855    [000] d..1.  7030.530293: cgroup_attach_task: dst_root=3 dst_id=22 dst_level=1 dst_path=/foreground pid=1900 comm=surfaceflinger
> >     PERFD-SERVER-1855    [000] d..1.  7030.530383: cgroup_attach_task: dst_root=3 dst_id=22 dst_level=1 dst_path=/foreground pid=1900 comm=surfaceflinger
> >    TimerDispatch-2768    [002] d..5.  7030.537542: sched_migrate_task: comm=surfaceflinger pid=1900 prio=98 orig_cpu=0 dest_cpu=4
> > <snip>
> > 
> > "A moving time" depends on how fast synchronize_rcu() completes. See
> > the first trace line. The migration has not occurred until the sync
> > was done first. Please note, number of different callbacks to be
> > invoked can be thousands.
> > 
> > 3. To address this drawback, maintain a separate track that consists
> > of synchronize_rcu() callers only. The GP-kthread, that drivers a GP
> > either wake-ups a worker to drain all list or directly wakes-up end
> > user if it is one in the drain list.
> > 
> 
> Late to the party, but I kinda wonder whether we can resolve it by:
> 
> 1) either introduce a separate seglist that only contains callbacks
> queued by call_rcu_hurry(), and whenever after an GP and callbacks are
> ready, call_rcu_hurry() callbacks will be called first.
> 
> 2) or make call_rcu_hurry() callbacks always inserted at the head of the
> NEXT list instead of the tail, e.g. (untested code):
> 
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index f71fac422c8f..89a875f8ecc7 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -338,13 +338,21 @@ bool rcu_segcblist_nextgp(struct rcu_segcblist *rsclp, unsigned long *lp)
>   * absolutely not OK for it to ever miss posting a callback.
>   */
>  void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> -			   struct rcu_head *rhp)
> +			   struct rcu_head *rhp,
> +			   bool is_lazy)
>  {
>  	rcu_segcblist_inc_len(rsclp);
>  	rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
> -	rhp->next = NULL;
> -	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
> -	WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], &rhp->next);
> +	/* If hurry and the list is not empty, put it in the front */
> +	if (!is_lazy && rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL) > 1) {
> +		// hurry callback, queued at front
> +		rhp->next = READ_ONCE(*rsclp->tails[RCU_NEXT_READY_TAIL]);
> +		WRITE_ONCE(*rsclp->tails[RCU_NEXT_READY_TAIL], rhp);
> +	} else {
> +		rhp->next = NULL;
> +		WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
> +		WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], &rhp->next);
> +	}
>  }
>  
>  /*
> diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> index 4fe877f5f654..459475bb8df9 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -136,7 +136,8 @@ struct rcu_head *rcu_segcblist_first_cb(struct rcu_segcblist *rsclp);
>  struct rcu_head *rcu_segcblist_first_pend_cb(struct rcu_segcblist *rsclp);
>  bool rcu_segcblist_nextgp(struct rcu_segcblist *rsclp, unsigned long *lp);
>  void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> -			   struct rcu_head *rhp);
> +			   struct rcu_head *rhp,
> +			   bool is_lazy);
>  bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
>  			   struct rcu_head *rhp);
>  void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 20d7a238d675..53adf5ab9c9f 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1241,7 +1241,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
>  		sdp = raw_cpu_ptr(ssp->sda);
>  	spin_lock_irqsave_sdp_contention(sdp, &flags);
>  	if (rhp)
> -		rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> +		rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp, true);
>  	rcu_segcblist_advance(&sdp->srcu_cblist,
>  			      rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
>  	s = rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq);
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 8d65f7d576a3..7dec7c68f88f 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -362,7 +362,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
>  	}
>  	if (needwake)
>  		rtpcp->urgent_gp = 3;
> -	rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
> +	rcu_segcblist_enqueue(&rtpcp->cblist, rhp, true);
>  	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
>  	if (unlikely(needadjust)) {
>  		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index cb1caefa8bd0..e05cbff40dc7 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2670,7 +2670,7 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
>  	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy))
>  		return; // Enqueued onto ->nocb_bypass, so just leave.
>  	// If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
> -	rcu_segcblist_enqueue(&rdp->cblist, head);
> +	rcu_segcblist_enqueue(&rdp->cblist, head, lazy_in);
>  	if (__is_kvfree_rcu_offset((unsigned long)func))
>  		trace_rcu_kvfree_callback(rcu_state.name, head,
>  					 (unsigned long)func,
> 
> Sure, there may be some corner cases I'm missing, but I think overall
> this is better than (sorta) duplicating the logic of seglist (the llist
> in sr_normal_state) or the logic of wake_rcu_gp()
> (synchronize_rcu_normal).
> 
> Anyway, these are just if-you-have-time-to-try options ;-)
> 
Hm.. You still mix callbacks and there is a dependency in order
of execution. The callback process time also might be varied from
one callback to another.

If you have many *_hurry() calls we end in the same situation. Apart
of that we also have !CONFIG_RCU_NOCB_CPU path that is also covered
by the patch that is in question.

--
Uladzislau Rezki