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McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 09/14] rcu: Speed up expedited GPs when interrupting RCU reader Date: Sun, 11 Nov 2018 11:28:59 -0800 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20181111192839.GA32144@linux.ibm.com> References: <20181111192839.GA32144@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18111119-0052-0000-0000-0000035421B3 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010028; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116014; UDB=6.00578730; IPR=6.00896114; MB=3.00024114; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-11 19:29:10 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111119-0053-0000-0000-00005EBB1CAE Message-Id: <20181111192904.3199-9-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-11_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=3 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1811110185 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In PREEMPT kernels, an expedited grace period might send an IPI to a CPU that is executing an RCU read-side critical section. In that case, it would be nice if the rcu_read_unlock() directly interacted with the RCU core code to immediately report the quiescent state. And this does happen in the case where the reader has been preempted. But it would also be a nice performance optimization if immediate reporting also happened in the preemption-free case. This commit therefore adds an ->exp_hint field to the task_struct structure's ->rcu_read_unlock_special field. The IPI handler sets this hint when it has interrupted an RCU read-side critical section, and this causes the outermost rcu_read_unlock() call to invoke rcu_read_unlock_special(), which, if preemption is enabled, reports the quiescent state immediately. If preemption is disabled, then the report is required to be deferred until preemption (or bottom halves or interrupts or whatever) is re-enabled. Because this is a hint, it does nothing for more complicated cases. For example, if the IPI interrupts an RCU reader, but interrupts are disabled across the rcu_read_unlock(), but another rcu_read_lock() is executed before interrupts are re-enabled, the hint will already have been cleared. If you do crazy things like this, reporting will be deferred until some later RCU_SOFTIRQ handler, context switch, cond_resched(), or similar. Reported-by: Joel Fernandes Signed-off-by: Paul E. McKenney Acked-by: Joel Fernandes (Google) --- include/linux/sched.h | 4 +++- kernel/rcu/tree_exp.h | 4 +++- kernel/rcu/tree_plugin.h | 14 +++++++++++--- 3 files changed, 17 insertions(+), 5 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index a51c13c2b1a0..e4c7b6241088 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -572,8 +572,10 @@ union rcu_special { struct { u8 blocked; u8 need_qs; + u8 exp_hint; /* Hint for performance. */ + u8 pad; /* No garbage from compiler! */ } b; /* Bits. */ - u16 s; /* Set of bits. */ + u32 s; /* Set of bits. */ }; enum perf_event_task_context { diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index e669ccf3751b..928fe5893a57 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -692,8 +692,10 @@ static void sync_rcu_exp_handler(void *unused) */ if (t->rcu_read_lock_nesting > 0) { raw_spin_lock_irqsave_rcu_node(rnp, flags); - if (rnp->expmask & rdp->grpmask) + if (rnp->expmask & rdp->grpmask) { rdp->deferred_qs = true; + WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true); + } raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 05915e536336..618956cc7a55 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -642,13 +642,21 @@ static void rcu_read_unlock_special(struct task_struct *t) local_irq_save(flags); irqs_were_disabled = irqs_disabled_flags(flags); - if ((preempt_bh_were_disabled || irqs_were_disabled) && - t->rcu_read_unlock_special.b.blocked) { + if (preempt_bh_were_disabled || irqs_were_disabled) { + WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); /* Need to defer quiescent state until everything is enabled. */ - raise_softirq_irqoff(RCU_SOFTIRQ); + if (irqs_were_disabled) { + /* Enabling irqs does not reschedule, so... */ + raise_softirq_irqoff(RCU_SOFTIRQ); + } else { + /* Enabling BH or preempt does reschedule, so... */ + set_tsk_need_resched(current); + set_preempt_need_resched(); + } local_irq_restore(flags); return; } + WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); rcu_preempt_deferred_qs_irqrestore(t, flags); } -- 2.17.1