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[73.148.104.166]) by smtp.gmail.com with ESMTPSA id v6-20020ae9e306000000b006b58fce19dasm14333694qkf.20.2022.07.19.15.42.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 Jul 2022 15:42:02 -0700 (PDT) Message-ID: <530b3bf4-f790-2973-5bc5-6ccd4797ae2a@joelfernandes.org> Date: Tue, 19 Jul 2022 18:42:00 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.0.2 Subject: Re: [PATCH rcu 4/7] rcu/nocb: Add an option to offload all CPUs on boot To: paulmck@kernel.org, Neeraj Upadhyay Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, Kalesh Singh , Uladzislau Rezki , kernel test robot References: <20220620224455.GA3840881@paulmck-ThinkPad-P17-Gen-1> <20220620224503.3841196-4-paulmck@kernel.org> <7a09896b-88dd-040b-86c0-863f8279a04e@quicinc.com> <20220719181257.GR1790663@paulmck-ThinkPad-P17-Gen-1> Content-Language: en-US From: Joel Fernandes In-Reply-To: <20220719181257.GR1790663@paulmck-ThinkPad-P17-Gen-1> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/19/2022 2:12 PM, Paul E. McKenney wrote: > On Tue, Jul 19, 2022 at 03:04:07PM +0530, Neeraj Upadhyay wrote: >> >> >> On 6/21/2022 4:15 AM, Paul E. McKenney wrote: >>> From: Joel Fernandes >>> >>> Systems built with CONFIG_RCU_NOCB_CPU=y but booted without either >>> the rcu_nocbs= or rcu_nohz_full= kernel-boot parameters will not have >>> callback offloading on any of the CPUs, nor can any of the CPUs be >>> switched to enable callback offloading at runtime. Although this is >>> intentional, it would be nice to have a way to offload all the CPUs >>> without having to make random bootloaders specify either the rcu_nocbs= >>> or the rcu_nohz_full= kernel-boot parameters. >>> >>> This commit therefore provides a new CONFIG_RCU_NOCB_CPU_DEFAULT_ALL >>> Kconfig option that switches the default so as to offload callback >>> processing on all of the CPUs. This default can still be overridden >>> using the rcu_nocbs= and rcu_nohz_full= kernel-boot parameters. >>> >>> Reviewed-by: Kalesh Singh >>> Reviewed-by: Uladzislau Rezki >>> (In v4.1, fixed issues with CONFIG maze reported by kernel test robot). >>> Reported-by: kernel test robot >>> Signed-off-by: Joel Fernandes >>> Signed-off-by: Paul E. McKenney >>> --- >> >> >> Reviewed-by: Neeraj Upadhyay >> >> One query on cpumask_setall() below >> >>> Documentation/admin-guide/kernel-parameters.txt | 6 ++++++ >>> kernel/rcu/Kconfig | 13 +++++++++++++ >>> kernel/rcu/tree_nocb.h | 15 ++++++++++++++- >>> 3 files changed, 33 insertions(+), 1 deletion(-) >>> >>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt >>> index 2522b11e593f2..34605c275294c 100644 >>> --- a/Documentation/admin-guide/kernel-parameters.txt >>> +++ b/Documentation/admin-guide/kernel-parameters.txt >>> @@ -3659,6 +3659,9 @@ >>> just as if they had also been called out in the >>> rcu_nocbs= boot parameter. >>> + Note that this argument takes precedence over >>> + the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option. >>> + >>> noiotrap [SH] Disables trapped I/O port accesses. >>> noirqdebug [X86-32] Disables the code which attempts to detect and >>> @@ -4557,6 +4560,9 @@ >>> no-callback mode from boot but the mode may be >>> toggled at runtime via cpusets. >>> + Note that this argument takes precedence over >>> + the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option. >>> + >>> rcu_nocb_poll [KNL] >>> Rather than requiring that offloaded CPUs >>> (specified by rcu_nocbs= above) explicitly >>> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig >>> index 1c630e573548d..27aab870ae4cf 100644 >>> --- a/kernel/rcu/Kconfig >>> +++ b/kernel/rcu/Kconfig >>> @@ -262,6 +262,19 @@ config RCU_NOCB_CPU >>> Say Y here if you need reduced OS jitter, despite added overhead. >>> Say N here if you are unsure. >>> +config RCU_NOCB_CPU_DEFAULT_ALL >>> + bool "Offload RCU callback processing from all CPUs by default" >>> + depends on RCU_NOCB_CPU >>> + default n >>> + help >>> + Use this option to offload callback processing from all CPUs >>> + by default, in the absence of the rcu_nocbs or nohz_full boot >>> + parameter. This also avoids the need to use any boot parameters >>> + to achieve the effect of offloading all CPUs on boot. >>> + >>> + Say Y here if you want offload all CPUs by default on boot. >>> + Say N here if you are unsure. >>> + >>> config TASKS_TRACE_RCU_READ_MB >>> bool "Tasks Trace RCU readers use memory barriers in user and idle" >>> depends on RCU_EXPERT && TASKS_TRACE_RCU >>> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h >>> index 4cf9a29bba79d..60cc92cc66552 100644 >>> --- a/kernel/rcu/tree_nocb.h >>> +++ b/kernel/rcu/tree_nocb.h >>> @@ -1197,11 +1197,21 @@ void __init rcu_init_nohz(void) >>> { >>> int cpu; >>> bool need_rcu_nocb_mask = false; >>> + bool offload_all = false; >>> struct rcu_data *rdp; >>> +#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) >>> + if (!rcu_state.nocb_is_setup) { >>> + need_rcu_nocb_mask = true; >>> + offload_all = true; >>> + } >>> +#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */ >>> + >>> #if defined(CONFIG_NO_HZ_FULL) >>> - if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) >>> + if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) { >>> need_rcu_nocb_mask = true; >>> + offload_all = false; /* NO_HZ_FULL has its own mask. */ >>> + } >>> #endif /* #if defined(CONFIG_NO_HZ_FULL) */ >>> if (need_rcu_nocb_mask) { >>> @@ -1222,6 +1232,9 @@ void __init rcu_init_nohz(void) >>> cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask); >>> #endif /* #if defined(CONFIG_NO_HZ_FULL) */ >>> + if (offload_all) >>> + cpumask_setall(rcu_nocb_mask); >> >> Do we need to do a cpumask_and(rcu_nocb_mask, cpu_possible_mask, >> rcu_nocb_mask) after setting all cpus in rcu_nocb_mask (cpumask_subset() >> check below takes care of it though)? > > Without that cpumask_and(), systems with sparse CPU numbering schemes > (for example, 0, 4, 8, 12, ...) will get a pr_info(), and as you noted, > the needed cpumask_and(). > > I am inclined to see a complaint before we change this. And perhaps if > this is to change, the change should be in cpumask_setall() rather than > in rcu_init_nohz(). But that is an argument for later, if at all. ;-) > >>> + >>> if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) { We could also suppress the pr_info() by making it conditional. like: if (!CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) { pr_info(...); } In other words, we could make the cpumask_and() as expected/normal on systems with sparse CPU numbering schemes. Would that work? Thanks, - Joel >>> pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n"); >>> cpumask_and(rcu_nocb_mask, cpu_possible_mask,