lkml.org 
[lkml]   [2023]   [Mar]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [PATCH 1/1] Reduce synchronize_rcu() waiting time
    On Tue, Mar 21, 2023 at 02:03:19PM +0000, Zhuo, Qiuxu wrote:
    > > From: Uladzislau Rezki (Sony) <urezki@gmail.com>
    > > 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

    \
     
     \ /
      Last update: 2023-03-27 01:11    [W:4.538 / U:0.452 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site