linux-kernel - Re: [PATCH tip/core/rcu 06/10] trace: Eliminate cond_resched_rcu_qs() in favor of cond

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Date:   Thu, 1 Mar 2018 00:04:04 -0500
From:   Steven Rostedt <rostedt@...dmis.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, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH tip/core/rcu 06/10] trace: Eliminate
 cond_resched_rcu_qs() in favor of cond_resched()

On Wed, 28 Feb 2018 17:21:44 -0800
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> > Perhaps, still think this is a special case. That said, perhaps
> > cond_resched isn't done in critical locations as it's a place that is
> > explicitly stating that it's OK to schedule.  
> 
> Building on your second sentence, when you are running a non-production
> stress test, adding an extra function call and conditional branch to
> cond_resched() should not be a problem.
> 
> So how about the (still untested) patch below?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit e9a6ea9fc2542459f9a63cf2b3a0264d09fbc266
> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Date:   Sun Feb 25 10:40:44 2018 -0800
> 
>     EXP sched: Make non-production PREEMPT cond_resched() help Tasks RCU
>     
>     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.
>     
>     Reported-by: Steven Rostedt <rostedt@...dmis.org>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index b161ef8a902e..970dadefb86f 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1589,6 +1589,12 @@ 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);

The thing I hate about this is that it is invasive to code outside of
the tracepoint benchmark. Why do the rcu_note_voluntary_context_switch
here and not in the tracepoint code? Seems odd to have it called
everywhere in the kernel when it is only needed by the benchmark
tracepoint code.

-- Steve



> +	return 0;
> +}
>  #else
>  static inline int _cond_resched(void) { return 0; }
>  #endif