Re: [PATCH 0/2] RCU offloading vs scheduler latency

[Uladzislau Rezki <urezki@gmail.com>]

> On Mon, May 02, 2022 at 09:08:31PM +0200, Uladzislau Rezki (Sony) wrote:
> > Motivation of backport:
> > -----------------------
> >    
> > 1. The cfcdef5e30469 ("rcu: Allow rcu_do_batch() to dynamically adjust batch sizes")
> > broke the default behaviour of "offloading rcu callbacks" setup. In that scenario
> > after each callback the caller context was used to check if it has to be rescheduled
> > giving a CPU time for others. After that change an "offloaded" setup can switch to
> > time-based RCU callbacks processing, what can be long for latency sensitive workloads
> > and SCHED_FIFO processes, i.e. callbacks are invoked for a long time with keeping
> > preemption off and without checking cond_resched().
> >     
> > 2. Our devices which run Android and 5.10 kernel have some critical areas which
> > are sensitive to latency. It is a low latency audio, 8k video, UI stack and so on.
> > For example below is a trace that illustrates a delay of "irq/396-5-0072" RT task
> > to complete IRQ processing:
> >     
> > <snip>
> >   rcuop/6-54  [000] d.h2  183.752989: irq_handler_entry:    irq=85 name=i2c_geni
> >   rcuop/6-54  [000] d.h5  183.753007: sched_waking:         comm=irq/396-5-0072 pid=12675 prio=49 target_cpu=000
> >   rcuop/6-54  [000] dNh6  183.753014: sched_wakeup:         irq/396-5-0072:12675 [49] success=1 CPU:000
> >   rcuop/6-54  [000] dNh2  183.753015: irq_handler_exit:     irq=85 ret=handled
> >   rcuop/6-54  [000] .N..  183.753018: rcu_invoke_callback:  rcu_preempt rhp=0xffffff88ffd440b0 func=__d_free.cfi_jt
> >   rcuop/6-54  [000] .N..  183.753020: rcu_invoke_callback:  rcu_preempt rhp=0xffffff892ffd8400 func=inode_free_by_rcu.cfi_jt
> >   rcuop/6-54  [000] .N..  183.753021: rcu_invoke_callback:  rcu_preempt rhp=0xffffff89327cd708 func=i_callback.cfi_jt
> >   ...
> >   rcuop/6-54  [000] .N..  183.755941: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c5a968 func=i_callback.cfi_jt
> >   rcuop/6-54  [000] .N..  183.755942: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c4bd20 func=__d_free.cfi_jt
> >   rcuop/6-54  [000] dN..  183.755944: rcu_batch_end:        rcu_preempt CBs-invoked=2112 idle=>c<>c<>c<>c<
> >   rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      Start context switch
> >   rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      End context switch
> >   rcuop/6-54  [000] d..2  183.755959: sched_switch:         rcuop/6:54 [120] R ==> migration/0:16 [0]
> >   ...
> >   migratio-16 [000] d..2  183.756021: sched_switch:         migration/0:16 [0] S ==> irq/396-5-0072:12675 [49]
> > <snip>
> >     
> > The "irq/396-5-0072:12675" was delayed for ~3 milliseconds due to introduced side effect.
> > Please note, on our Android devices we get ~70 000 callbacks registered to be invoked by
> > the "rcuop/x" workers. This is during 1 seconds time interval and regular handset usage.
> > Latencies bigger that 3 milliseconds affect our high-resolution audio streaming over the
> > LDAC/Bluetooth stack.
> > 
> > Two patches depend on each other.
> 
> One meta-comment.  We can't apply changes to older kernels and not newer
> ones, as you do not want to upgrade your kernel and suffer a regression.
> This patch series comes from 5.17, but you are backporting to only 5.10.
> What about 5.15?  I can't consider this series unless we have a series
> also for 5.15 for that reason, we have to keep in sync otherwise things
> get unmaintainable.
> 
> So, have a 5.15 backport as well?
> 
Yep, i can prepare a backport for 5.15 as well. So i can resend the patches 
for 5.10 and 5.15 stable kernels, in total there will be 4 patches with two
separate cover letters.

Does it work for you?

> Also, you forgot to cc: the developers of the patches in this 0/X email,
> that just causes confusion for those that do not receive this message.
> 
Sorry, i missed that point in cover latter. So will update it with appropriate
people on my next resend.

Thanks!

--
Uladzislau Rezki