linux-kernel - Re: [PATCH tip/core/rcu 01/22] sched: Make non-production PREEMPT cond

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Date:   Mon, 23 Apr 2018 10:51:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel.opensrc@...il.com
Subject: Re: [PATCH tip/core/rcu 01/22] sched: Make non-production PREEMPT
 cond_resched() help Tasks RCU

On Sun, Apr 22, 2018 at 07:32:06PM -0700, Paul E. McKenney wrote:
> In CONFIG_PREEMPT=y kernels, cond_resched() is a complete no-op, and
> thus cannot help advance Tasks-RCU grace periods.  However, such grace
> periods are only an issue in non-production benchmarking runs of the
> Linux kernel.  This commit therefore makes cond_resched() invoke
> rcu_note_voluntary_context_switch() for kernels implementing Tasks RCU
> even in CONFIG_PREEMPT=y kernels.

I'm confused.. why is having this conditional on TRACEPOINT_BENCHMARK a
sane idea?

> 
> Reported-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  include/linux/sched.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index b3d697f3b573..fe8f7178a22d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1617,6 +1617,12 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
>   */
>  #ifndef CONFIG_PREEMPT
>  extern int _cond_resched(void);
> +#elif defined(CONFIG_TRACEPOINT_BENCHMARK)
> +static inline int _cond_resched(void)
> +{
> +	rcu_note_voluntary_context_switch(current);
> +	return 0;
> +}
>  #else
>  static inline int _cond_resched(void) { return 0; }
>  #endif
> -- 
> 2.5.2
>