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McKenney" , Sasha Levin Subject: [PATCH 5.15 401/530] rcu: Avoid triggering strict-GP irq-work when RCU is idle Date: Mon, 24 Oct 2022 13:32:25 +0200 Message-Id: <20221024113103.228321513@linuxfoundation.org> X-Mailer: git-send-email 2.38.1 In-Reply-To: <20221024113044.976326639@linuxfoundation.org> References: <20221024113044.976326639@linuxfoundation.org> User-Agent: quilt/0.67 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: Zqiang [ Upstream commit 621189a1fe93cb2b34d62c5cdb9e258bca044813 ] Kernels built with PREEMPT_RCU=3Dy and RCU_STRICT_GRACE_PERIOD=3Dy trigger irq-work from rcu_read_unlock(), and the resulting irq-work handler invokes rcu_preempt_deferred_qs_handle(). The point of this triggering is to force grace periods to end quickly in order to give tools like KASAN a better chance of detecting RCU usage bugs such as leaking RCU-protected pointers out of an RCU read-side critical section. However, this irq-work triggering is unconditional. This works, but there is no point in doing this irq-work unless the current grace period is waiting on the running CPU or task, which is not the common case. After all, in the common case there are many rcu_read_unlock() calls per CPU per grace period. This commit therefore triggers the irq-work only when the current grace period is waiting on the running CPU or task. This change was tested as follows on a four-CPU system: echo rcu_preempt_deferred_qs_handler > /sys/kernel/debug/tracing/set_ftrac= e_filter echo 1 > /sys/kernel/debug/tracing/function_profile_enabled insmod rcutorture.ko sleep 20 rmmod rcutorture.ko echo 0 > /sys/kernel/debug/tracing/function_profile_enabled echo > /sys/kernel/debug/tracing/set_ftrace_filter This procedure produces results in this per-CPU set of files: /sys/kernel/debug/tracing/trace_stat/function* Sample output from one of these files is as follows: Function Hit Time Avg = s^2 -------- --- ---- --- = --- rcu_preempt_deferred_qs_handle 838746 182650.3 us 0.217 us = 0.004 us The baseline sum of the "Hit" values (the number of calls to this function) was 3,319,015. With this commit, that sum was 1,140,359, for a 2.9x reduction. The worst-case variance across the CPUs was less than 25%, so this large effect size is statistically significant. The raw data is available in the Link: URL. Link: https://lore.kernel.org/all/20220808022626.12825-1-qiang1.zhang@intel= .com/ Signed-off-by: Zqiang Signed-off-by: Paul E. McKenney Signed-off-by: Sasha Levin --- kernel/rcu/tree_plugin.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index ef2dd131e955..f1a73a1f8472 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -638,7 +638,8 @@ static void rcu_read_unlock_special(struct task_struct = *t) =20 expboost =3D (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_= tasks)) || (rdp->grpmask & READ_ONCE(rnp->expmask)) || - IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) || + (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && + ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node)) || (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && t->rcu_blocked_node); // Need to defer quiescent state until everything is enabled. --=20 2.35.1