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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v2] rcutorture: Create nocb tasks only for CONFIG_RCU_NOCB_CPU=y kernels
Date:   Thu, 2 Feb 2023 10:13:19 -0500
Message-Id: <7959A7ED-A47F-4472-9278-8582F1CCC6A9@joelfernandes.org>
References: <20230202070245.3311951-1-qiang1.zhang@intel.com>
Cc:     paulmck@kernel.org, frederic@kernel.org, quic_neeraju@quicinc.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org
In-Reply-To: <20230202070245.3311951-1-qiang1.zhang@intel.com>
To:     Zqiang <qiang1.zhang@intel.com>
Precedence: bulk


> On Feb 2, 2023, at 1:57 AM, Zqiang <qiang1.zhang@intel.com> wrote:
>=20
> =EF=BB=BFWhen setting nocbs_nthreads to start rcutorture test with a non-z=
ero value,
> the nocb tasks will be created and invoke rcu_nocb_cpu_offload/deoffload()=

> to toggle CPU's callback-offload state, but for CONFIG_RCU_NOCB_CPU=3Dn
> kernel, the rcu_nocb_cpu_offload/deoffload() is a no-op and this is also
> meaningless for torture_type is non-rcu.
>=20
> This commit therefore add member can_nocbs_toggle to rcu_torture_ops
> structure to avoid unnecessary nocb tasks creation.
>=20
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
> kernel/rcu/rcutorture.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)

Sorry if I am missing something but what is the point of adding more lines o=
f code and complexity to handle this? Does it improve the test coverage or r=
educe overhead?

This is test code. I see no problem with cost of an extra unused task with p=
ositive trade off of keeping the code simple=E2=80=A6

thanks,

 - Joel=20


>=20
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 297da28ce92d..ced0a8e1d765 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -383,6 +383,7 @@ struct rcu_torture_ops {
>    long cbflood_max;
>    int irq_capable;
>    int can_boost;
> +    int can_nocbs_toggle;
>    int extendables;
>    int slow_gps;
>    int no_pi_lock;
> @@ -569,6 +570,7 @@ static struct rcu_torture_ops rcu_ops =3D {
>    .stall_dur        =3D rcu_jiffies_till_stall_check,
>    .irq_capable        =3D 1,
>    .can_boost        =3D IS_ENABLED(CONFIG_RCU_BOOST),
> +    .can_nocbs_toggle    =3D IS_ENABLED(CONFIG_RCU_NOCB_CPU),
>    .extendables        =3D RCUTORTURE_MAX_EXTEND,
>    .name            =3D "rcu"
> };
> @@ -2356,7 +2358,7 @@ rcu_torture_print_module_parms(struct rcu_torture_op=
s *cur_ops, const char *tag)
>         "n_barrier_cbs=3D%d "
>         "onoff_interval=3D%d onoff_holdoff=3D%d "
>         "read_exit_delay=3D%d read_exit_burst=3D%d "
> -         "nocbs_nthreads=3D%d nocbs_toggle=3D%d "
> +         "nocbs_nthreads=3D%d/%d nocbs_toggle=3D%d "
>         "test_nmis=3D%d\n",
>         torture_type, tag, nrealreaders, nfakewriters,
>         stat_interval, verbose, test_no_idle_hz, shuffle_interval,
> @@ -2368,7 +2370,7 @@ rcu_torture_print_module_parms(struct rcu_torture_op=
s *cur_ops, const char *tag)
>         n_barrier_cbs,
>         onoff_interval, onoff_holdoff,
>         read_exit_delay, read_exit_burst,
> -         nocbs_nthreads, nocbs_toggle,
> +         nocbs_nthreads, cur_ops->can_nocbs_toggle, nocbs_toggle,
>         test_nmis);
> }
>=20
> @@ -3708,6 +3710,10 @@ rcu_torture_init(void)
>        pr_alert("rcu-torture: ->fqs NULL and non-zero fqs_duration, fqs di=
sabled.\n");
>        fqs_duration =3D 0;
>    }
> +    if (!cur_ops->can_nocbs_toggle && nocbs_nthreads !=3D 0) {
> +        pr_alert("rcu-torture: ->can_nocbs_toggle false and non-zero nocb=
s_nthreads, nocbs_toggle disabled.\n");
> +        nocbs_nthreads =3D 0;
> +    }
>    if (cur_ops->init)
>        cur_ops->init();
>=20
> --=20
> 2.25.1
>=20