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Date:   Mon, 20 Nov 2023 17:02:14 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com,
        Davidlohr Bueso <dave@...olabs.net>,
        Josh Triplett <josh@...htriplett.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [RFC PATCH 60/86] treewide: torture: remove cond_resched()

On Tue, Nov 07, 2023 at 03:07:56PM -0800, Ankur Arora wrote:
> Some cases changed to cond_resched_stall() to avoid changing
> the behaviour of the test too drastically.
> 
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>

Given lazy preemption, I am OK with dropping the cond_resched()
invocations from the various torture tests.

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/rcu/rcuscale.c   | 2 --
>  kernel/rcu/rcutorture.c | 8 ++++----
>  kernel/scftorture.c     | 1 -
>  kernel/torture.c        | 1 -
>  4 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
> index ffdb30495e3c..737620bbec83 100644
> --- a/kernel/rcu/rcuscale.c
> +++ b/kernel/rcu/rcuscale.c
> @@ -672,8 +672,6 @@ kfree_scale_thread(void *arg)
>  			else
>  				kfree_rcu(alloc_ptr, rh);
>  		}
> -
> -		cond_resched();
>  	} while (!torture_must_stop() && ++loop < kfree_loops);
>  
>  	if (atomic_inc_return(&n_kfree_scale_thread_ended) >= kfree_nrealthreads) {
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index ade42d6a9d9b..158d58710b51 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -81,7 +81,7 @@ torture_param(int, fqs_stutter, 3, "Wait time between fqs bursts (s)");
>  torture_param(int, fwd_progress, 1, "Number of grace-period forward progress tasks (0 to disable)");
>  torture_param(int, fwd_progress_div, 4, "Fraction of CPU stall to wait");
>  torture_param(int, fwd_progress_holdoff, 60, "Time between forward-progress tests (s)");
> -torture_param(bool, fwd_progress_need_resched, 1, "Hide cond_resched() behind need_resched()");
> +torture_param(bool, fwd_progress_need_resched, 1, "Hide cond_resched_stall() behind need_resched()");
>  torture_param(bool, gp_cond, false, "Use conditional/async GP wait primitives");
>  torture_param(bool, gp_cond_exp, false, "Use conditional/async expedited GP wait primitives");
>  torture_param(bool, gp_cond_full, false, "Use conditional/async full-state GP wait primitives");
> @@ -2611,7 +2611,7 @@ static void rcu_torture_fwd_prog_cond_resched(unsigned long iter)
>  		return;
>  	}
>  	// No userspace emulation: CB invocation throttles call_rcu()
> -	cond_resched();
> +	cond_resched_stall();
>  }
>  
>  /*
> @@ -2691,7 +2691,7 @@ static void rcu_torture_fwd_prog_nr(struct rcu_fwd *rfp,
>  		udelay(10);
>  		cur_ops->readunlock(idx);
>  		if (!fwd_progress_need_resched || need_resched())
> -			cond_resched();
> +			cond_resched_stall();
>  	}
>  	(*tested_tries)++;
>  	if (!time_before(jiffies, stopat) &&
> @@ -3232,7 +3232,7 @@ static int rcu_torture_read_exit(void *unused)
>  				errexit = true;
>  				break;
>  			}
> -			cond_resched();
> +			cond_resched_stall();
>  			kthread_stop(tsp);
>  			n_read_exits++;
>  		}
> diff --git a/kernel/scftorture.c b/kernel/scftorture.c
> index 59032aaccd18..24192fe01125 100644
> --- a/kernel/scftorture.c
> +++ b/kernel/scftorture.c
> @@ -487,7 +487,6 @@ static int scftorture_invoker(void *arg)
>  			set_cpus_allowed_ptr(current, cpumask_of(cpu));
>  			was_offline = false;
>  		}
> -		cond_resched();
>  		stutter_wait("scftorture_invoker");
>  	} while (!torture_must_stop());
>  
> diff --git a/kernel/torture.c b/kernel/torture.c
> index b28b05bbef02..0c0224c76275 100644
> --- a/kernel/torture.c
> +++ b/kernel/torture.c
> @@ -747,7 +747,6 @@ bool stutter_wait(const char *title)
>  			while (READ_ONCE(stutter_pause_test)) {
>  				if (!(i++ & 0xffff))
>  					torture_hrtimeout_us(10, 0, NULL);
> -				cond_resched();
>  			}
>  		} else {
>  			torture_hrtimeout_jiffies(round_jiffies_relative(HZ), NULL);
> -- 
> 2.31.1
> 

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