Re: [PATCH] rcu: Fix delayed execution of hurry callbacks

["Tze-nan Wu (吳澤南)" <Tze-nan.Wu@mediatek.com>]

On Thu, 2025-07-17 at 15:34 +0200, Frederic Weisbecker wrote:
> 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
> 
> 
> Le Thu, Jul 17, 2025 at 01:53:38PM +0800, Tze-nan Wu a écrit :
> > We observed a regression in our customer’s environment after
> > enabling
> > CONFIG_LAZY_RCU. In the Android Update Engine scenario, where
> > ioctl() is
> > used heavily, we found that callbacks queued via call_rcu_hurry
> > (such as
> > percpu_ref_switch_to_atomic_rcu) can sometimes be delayed by up to
> > 5
> > seconds before execution. This occurs because the new grace period
> > does
> > not start immediately after the previous one completes.
> > 
> > The root cause is that the wake_nocb_gp_defer() function now checks
> > "rdp->nocb_defer_wakeup" instead of "rdp_gp->nocb_defer_wakeup". On
> > CPUs
> > that are not rcuog, "rdp->nocb_defer_wakeup" may always be
> > RCU_NOCB_WAKE_NOT. This can cause "rdp_gp->nocb_defer_wakeup" to be
> > downgraded and the "rdp_gp->nocb_timer" to be postponed by up to 10
> > seconds, delaying the execution of hurry RCU callbacks.
> > 
> > The trace log of one scenario we encountered is as follow:
> >   // previous GP ends at this point
> >   rcu_preempt   [000] d..1.   137.240210: rcu_grace_period:
> > rcu_preempt 8369 end
> >   rcu_preempt   [000] .....   137.240212: rcu_grace_period:
> > rcu_preempt 8372 reqwait
> >   // call_rcu_hurry enqueues "percpu_ref_switch_to_atomic_rcu", the
> > callback waited on by UpdateEngine
> >   update_engine [002] d..1.   137.301593: __call_rcu_common: wyy:
> > unlikely p_ref = 00000000********. lazy = 0
> >   // FirstQ on cpu 2 rdp_gp->nocb_timer is set to fire after 1
> > jiffy (4ms)
> >   // and the rdp_gp->nocb_defer_wakeup is set to RCU_NOCB_WAKE
> >   update_engine [002] d..2.   137.301595: rcu_nocb_wake:
> > rcu_preempt 2 FirstQ on cpu2 with rdp_gp (cpu0).
> >   // FirstBQ event on cpu2 during the 1 jiffy, make the timer
> > postpond 10 seconds later.
> >   // also, the rdp_gp->nocb_defer_wakeup is overwrite to
> > RCU_NOCB_WAKE_LAZY
> >   update_engine [002] d..1.   137.301601: rcu_nocb_wake:
> > rcu_preempt 2 WakeEmptyIsDeferred
> >   ...
> >   ...
> >   ...
> >   // before the 10 seconds timeout, cpu0 received another
> > call_rcu_hurry
> >   // reset the timer to jiffies+1 and set the waketype =
> > RCU_NOCB_WAKE.
> >   kworker/u32:0 [000] d..2.   142.557564: rcu_nocb_wake:
> > rcu_preempt 0 FirstQ
> >   kworker/u32:0 [000] d..1.   142.557576: rcu_nocb_wake:
> > rcu_preempt 0 WakeEmptyIsDeferred
> >   kworker/u32:0 [000] d..1.   142.558296: rcu_nocb_wake:
> > rcu_preempt 0 WakeNot
> >   kworker/u32:0 [000] d..1.   142.558562: rcu_nocb_wake:
> > rcu_preempt 0 WakeNot
> >   // idle(do_nocb_deferred_wakeup) wake rcuog due to waketype ==
> > RCU_NOCB_WAKE
> >   <idle>        [000] d..1.   142.558786: rcu_nocb_wake:
> > rcu_preempt 0 DoWake
> >   <idle>        [000] dN.1.   142.558839: rcu_nocb_wake:
> > rcu_preempt 0 DeferredWake
> >   rcuog/0       [000] .....   142.558871: rcu_nocb_wake:
> > rcu_preempt 0 EndSleep
> >   rcuog/0       [000] .....   142.558877: rcu_nocb_wake:
> > rcu_preempt 0 Check
> >   // finally rcuog request a new GP at this point (5 seconds after
> > the FirstQ event)
> >   rcuog/0       [000] d..2.   142.558886: rcu_grace_period:
> > rcu_preempt 8372 newreq
> >   rcu_preempt   [001] d..1.   142.559458: rcu_grace_period:
> > rcu_preempt 8373 start
> >   ...
> >   rcu_preempt   [000] d..1.   142.564258: rcu_grace_period:
> > rcu_preempt 8373 end
> >   rcuop/2       [000] D..1.   142.566337: rcu_batch_start:
> > rcu_preempt CBs=219 bl=10
> >   // the hurry CB is invoked at this point
> >   rcuop/2       [000] b....   142.566352:
> > blk_queue_usage_counter_release: wyy: wakeup. p_ref =
> > 00000000********.
> > 
> > This patch changes the condition to check "rdp_gp-
> > >nocb_defer_wakeup" in
> > the lazy path. This prevents an already scheduled "rdp_gp-
> > >nocb_timer"
> > from being postponed and avoids overwriting "rdp_gp-
> > >nocb_defer_wakeup"
> > when it is not RCU_NOCB_WAKE_NOT.
> > 
> > Fixes: 3cb278e73be5 ("rcu: Make call_rcu() lazy to save power")
> > Co-developed-by: Cheng-jui Wang <cheng-jui.wang@mediatek.com>
> > Signed-off-by: Cheng-jui Wang <cheng-jui.wang@mediatek.com>
> > Co-developed-by: Lorry.Luo@mediatek.com
> > Signed-off-by: Lorry.Luo@mediatek.com
> > Tested-by: weiyangyang@vivo.com
> > Signed-off-by: weiyangyang@vivo.com
> > Signed-off-by: Tze-nan Wu <Tze-nan.Wu@mediatek.com>
> > ---
> > The regression is first observed by wyy in the Update Engine
> > scenario
> > with  CONFIG_LAZY_RCU enabled. there is an additional delay of 4–5
> > seconds during the heavy ioctl API call, waiting for
> > percpu_ref_switch_to_atomic_rcu (RCU hurry CB) to complete.
> > 
> > Initially, we suspected that the percpu_ref_switch_to_atomic_rcu
> > function itself was taking too long. However, after enabling some
> > custome and the following trace events: rcu_do_batch,
> > rcu_nocb_wake, and
> > rcu_grace_period. we found that the root cause was that rcuog was
> > not
> > being woken up in time to request a new GP. This led to the delay
> > in
> > invoking the hurry RCU callback (percpu_ref_switch_to_atomic_rcu).
> > 
> > Environment:
> >   Android-16, Kernel: 6.12, 8 CPUs (ARM)
> > 
> > Configuration:
> >   CONFIG_TREE_RCU=y
> >   CONFIG_PREEMPT_RCU=y
> >   CONFIG_LAZY_RCU=y
> >   CONFIG_RCU_NOCB_CPU=y
> >   CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y
> >   rcu_nocb_gp_stride = -1 (default is 4 for 8 cores)
> >   jiffies_lazy_flush = 10 * HZ
> > 
> > Contributions:
> > Tze-Nan Wu:
> > Collaborated with Cheng-Jui to discuss which tracepoints needed to
> > be
> > added, jointly analyzed the trace logs, identified the root cause,
> > and
> > proposed this upstream change.
> > 
> > Cheng-Jui Wang:
> > Provided many valuable suggestions during the debugging process,
> > repeatedly found breakthroughs when we were stuck, and helped
> > identify
> > the root cause.
> > 
> > Lorry Luo:
> > Assisted in verifying whether rcu-hurry-callback was executed too
> > long
> > or deferred, supported with testing, and helped with communication.
> > 
> > Weiyangyang:
> > Main tester who discovered the regression scenario, confirmed that
> > enabling CONFIG_LAZY_RCU caused the regression, and verified that
> > this
> > patch resolves the issue
> 
> Nice team work :-)
> 
Thanks :)
> > 
> > Note:
> > With my limited understanding of lazy RCU, I am not fully confident
> > that
> > this is a real issue. In my opinion, hurry callbacks should not be
> > delayed by other events such as firstBQ trace event.
> > If my understanding is incorrect, I would greatly appreciate any
> > guidance or clarification from the maintainers.
> 
> Your understanding looks quite right!
> 
Glad to know I was on the right track.

> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> 
> Since the issue is there for 3 years now and was introduced with
> the CONFIG_LAZY_RCU new feature, it can probably wait a few weeks
> for the next merge window.
> 
> --
> Frederic Weisbecker
> SUSE Labs

Totally understand, we'll follow the kernel's development schedule.
--
Tze-Nan Wu