On Tue, Mar 21, 2023 at 02:03:19PM +0000, Zhuo, Qiuxu wrote: > > From: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> > > Sent: Tuesday, March 21, 2023 6:28 PM > > [...] > > Subject: [PATCH 1/1] Reduce synchronize_rcu() waiting time > > > > A call to a synchronize_rcu() can be expensive from time point of view. > > Different workloads can be affected by this especially the ones which use this > > API in its time critical sections. > > > > This is interesting and meaningful research. ;-) > > > For example in case of NOCB scenario the wakeme_after_rcu() callback > > invocation depends on where in a nocb-list it is located. Below is an example > > when it was the last out of ~3600 callbacks: > > > > <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> > > > > Did the results above tell us that CBs-invoked=3612 during the time 21950.145313 ~ 21950.152625? > Yes. > > If possible, may I know the steps, commands, and related parameters to produce the results above? > Thank you! > Build the kernel with CONFIG_RCU_TRACE configuration. Update your "set_event" file with appropriate traces: <snip> XQ-DQ54:/sys/kernel/tracing # echo rcu:rcu_batch_start rcu:rcu_batch_end rcu:rcu_invoke_callback > set_event XQ-DQ54:/sys/kernel/tracing # cat set_event rcu:rcu_batch_start rcu:rcu_invoke_callback rcu:rcu_batch_end XQ-DQ54:/sys/kernel/tracing # <snip> Collect traces as much as you want: XQ-DQ54:/sys/kernel/tracing # echo 1 > tracing_on; sleep 10; echo 0 > tracing_on Next problem is how to parse it. Of course you will not be able to parse megabytes of traces. For that purpose i use a special C trace parser. If you need an example please let me know i can show here. -- Uladzislau Rezki
