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    SubjectRe: [PATCH 1/1] Reduce synchronize_rcu() waiting time
    On Mon, Mar 27, 2023 at 11:21:23AM +0000, Zhang, Qiang1 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:
    > >
    >
    >
    >
    > Can it be implemented separately as follows? it seems that the code is simpler
    > (only personal opinion) 😊.
    >
    > But I didn't test whether this reduce synchronize_rcu() waiting time
    >
    > +static void rcu_poll_wait_gp(struct rcu_tasks *rtp)
    > +{
    > + unsigned long gp_snap;
    > +
    > + gp_snap = start_poll_synchronize_rcu();
    > + while (!poll_state_synchronize_rcu(gp_snap))
    > + schedule_timeout_idle(1);

    I could be wrong, but my guess is that the guys working with
    battery-powered devices are not going to be very happy with this loop.

    All those wakeups by all tasks waiting for a grace period end up
    consuming a surprisingly large amount of energy.

    Thanx, Paul

    > +}
    > +
    > +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func);
    > +DEFINE_RCU_TASKS(rcu_poll, rcu_poll_wait_gp, call_rcu_poll,
    > + "RCU Poll");
    > +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func)
    > +{
    > + call_rcu_tasks_generic(rhp, func, &rcu_poll);
    > +}
    > +EXPORT_SYMBOL_GPL(call_rcu_poll);
    > +
    > +void synchronize_rcu_poll(void)
    > +{
    > + synchronize_rcu_tasks_generic(&rcu_poll);
    > +}
    > +EXPORT_SYMBOL_GPL(synchronize_rcu_poll);
    > +
    > +static int __init rcu_spawn_poll_kthread(void)
    > +{
    > + cblist_init_generic(&rcu_poll);
    > + rcu_poll.gp_sleep = HZ / 10;
    > + rcu_spawn_tasks_kthread_generic(&rcu_poll);
    > + return 0;
    > +}
    >
    > Thanks
    > Zqiang
    >
    >
    > > >
    > > > <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

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