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McKenney" To: Joel Fernandes Cc: Uladzislau Rezki , rcu@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed Message-ID: <20221102163544.GM5600@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20221029132856.3752018-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 02, 2022 at 12:13:17PM -0400, Joel Fernandes wrote: > On Wed, Nov 2, 2022 at 8:37 AM Uladzislau Rezki wrote: > > > > On Sat, Oct 29, 2022 at 01:28:56PM +0000, Joel Fernandes (Google) wrote: > > > On ChromeOS, I am (almost) always seeing the optimization trigger. > > > Tested boot up and trace_printk'ing how often it triggers. > > > > > > Signed-off-by: Joel Fernandes (Google) > > > --- > > > kernel/rcu/tree.c | 18 +++++++++++++++++- > > > 1 file changed, 17 insertions(+), 1 deletion(-) > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 591187b6352e..3e4c50b9fd33 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -2935,6 +2935,7 @@ struct kfree_rcu_cpu_work { > > > > > > /** > > > * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period > > > + * @rdp: The rdp of the CPU that this kfree_rcu corresponds to. > > > * @head: List of kfree_rcu() objects not yet waiting for a grace period > > > * @bkvhead: Bulk-List of kvfree_rcu() objects not yet waiting for a grace period > > > * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period > > > @@ -2964,6 +2965,8 @@ struct kfree_rcu_cpu { > > > struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES]; > > > raw_spinlock_t lock; > > > struct delayed_work monitor_work; > > > + struct rcu_data *rdp; > > > + unsigned long last_gp_seq; > > > bool initialized; > > > int count; > > > > > > @@ -3167,6 +3170,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp) > > > mod_delayed_work(system_wq, &krcp->monitor_work, delay); > > > return; > > > } > > > + krcp->last_gp_seq = krcp->rdp->gp_seq; > > > queue_delayed_work(system_wq, &krcp->monitor_work, delay); > > > } > > > > > > @@ -3217,7 +3221,17 @@ static void kfree_rcu_monitor(struct work_struct *work) > > > // be that the work is in the pending state when > > > // channels have been detached following by each > > > // other. > > > - queue_rcu_work(system_wq, &krwp->rcu_work); > > > + // > > > + // NOTE about gp_seq wrap: In case of gp_seq overflow, > > > + // it is possible for rdp->gp_seq to be less than > > > + // krcp->last_gp_seq even though a GP might be over. In > > > + // this rare case, we would just have one extra GP. > > > + if (krcp->last_gp_seq && > > > > > This check can be eliminated i think. A kfree_rcu_cpu is defined as > > static so by default the last_gp_set is set to zero. > > Ack. > > > > @@ -4802,6 +4816,8 @@ static void __init kfree_rcu_batch_init(void) > > > for_each_possible_cpu(cpu) { > > > struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu); > > > > > > + krcp->rdp = per_cpu_ptr(&rcu_data, cpu); > > > + krcp->last_gp_seq = 0; > > > > > Yep. This one can be just dropped. > > > > But all the rest looks good :) I will give it a try from test point of > > view. It is interested from the memory footprint point of view. > > Ack. Thanks. Even though we should not sample rdp->gp_seq, I think it > is still worth a test. Just for completeness, the main purpose of rdp->gp_seq is to reject quiescent states that were seen during already-completed grace periods. Thanx, Paul