On Tue, May 03, 2022 at 09:17:07PM +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.
All now queued up, thanks.
greg k-h
