From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
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
Subject: Re: [PATCH RFC tip/core/rcu 02/14] rcu/nocb: Add bypass callback queueing
Date: Tue, 6 Aug 2019 20:16:16 -0400 [thread overview]
Message-ID: <20190807001616.GA169551@google.com> (raw)
In-Reply-To: <20190807000313.GA161170@google.com>
On Tue, Aug 06, 2019 at 08:03:13PM -0400, Joel Fernandes wrote:
> On Fri, Aug 02, 2019 at 08:14:49AM -0700, Paul E. McKenney wrote:
> > Use of the rcu_data structure's segmented ->cblist for no-CBs CPUs
> > takes advantage of unrelated grace periods, thus reducing the memory
> > footprint in the face of floods of call_rcu() invocations. However,
> > the ->cblist field is a more-complex rcu_segcblist structure which must
> > be protected via locking. Even though there are only three entities
> > which can acquire this lock (the CPU invoking call_rcu(), the no-CBs
> > grace-period kthread, and the no-CBs callbacks kthread), the contention
> > on this lock is excessive under heavy stress.
> >
> > This commit therefore greatly reduces contention by provisioning
> > an rcu_cblist structure field named ->nocb_bypass within the
> > rcu_data structure. Each no-CBs CPU is permitted only a limited
> > number of enqueues onto the ->cblist per jiffy, controlled by a new
> > nocb_nobypass_lim_per_jiffy kernel boot parameter that defaults to
> > about 16 enqueues per millisecond (16 * 1000 / HZ). When that limit is
> > exceeded, the CPU instead enqueues onto the new ->nocb_bypass.
>
> Looks quite interesting. I am guessing the not-no-CB (regular) enqueues don't
> need to use the same technique because both enqueues / callback execution are
> happening on same CPU..
>
> Still looking through patch but I understood the basic idea. Some nits below:
>
> [snip]
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 2c3e9068671c..e4df86db8137 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -200,18 +200,26 @@ struct rcu_data {
> > atomic_t nocb_lock_contended; /* Contention experienced. */
> > int nocb_defer_wakeup; /* Defer wakeup of nocb_kthread. */
> > struct timer_list nocb_timer; /* Enforce finite deferral. */
> > + unsigned long nocb_gp_adv_time; /* Last call_rcu() CB adv (jiffies). */
> > +
> > + /* The following fields are used by call_rcu, hence own cacheline. */
> > + raw_spinlock_t nocb_bypass_lock ____cacheline_internodealigned_in_smp;
> > + struct rcu_cblist nocb_bypass; /* Lock-contention-bypass CB list. */
> > + unsigned long nocb_bypass_first; /* Time (jiffies) of first enqueue. */
> > + unsigned long nocb_nobypass_last; /* Last ->cblist enqueue (jiffies). */
> > + int nocb_nobypass_count; /* # ->cblist enqueues at ^^^ time. */
>
> Can these and below fields be ifdef'd out if !CONFIG_RCU_NOCB_CPU so as to
> keep the size of struct smaller for benefit of systems that don't use NOCB?
>
>
> >
> > /* The following fields are used by GP kthread, hence own cacheline. */
> > raw_spinlock_t nocb_gp_lock ____cacheline_internodealigned_in_smp;
> > - bool nocb_gp_sleep;
> > - /* Is the nocb GP thread asleep? */
> > + struct timer_list nocb_bypass_timer; /* Force nocb_bypass flush. */
> > + bool nocb_gp_sleep; /* Is the nocb GP thread asleep? */
>
> And these too, I think.
Please ignore this comment, I missed that these were already ifdef'd out
since it did not appear in the diff.
thanks!
next prev parent reply other threads:[~2019-08-07 0:16 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-02 15:14 [PATCH tip/core/rcu 0/14] No-CBs bypass addition for v5.4 Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in rcu_segcblist structure Paul E. McKenney
2019-08-04 14:50 ` Peter Zijlstra
2019-08-04 14:52 ` Peter Zijlstra
2019-08-04 18:45 ` Paul E. McKenney
2019-08-04 18:42 ` Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 02/14] rcu/nocb: Add bypass callback queueing Paul E. McKenney
2019-08-07 0:03 ` Joel Fernandes
2019-08-07 0:16 ` Joel Fernandes [this message]
2019-08-07 0:35 ` Paul E. McKenney
2019-08-07 0:40 ` Steven Rostedt
2019-08-07 1:17 ` Paul E. McKenney
2019-08-07 1:24 ` Steven Rostedt
2019-08-07 3:47 ` Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 03/14] rcu/nocb: EXP Check use and usefulness of ->nocb_lock_contended Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 04/14] rcu/nocb: Print no-CBs diagnostics when rcutorture writer unduly delayed Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 05/14] rcu/nocb: Avoid synchronous wakeup in __call_rcu_nocb_wake() Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 06/14] rcu/nocb: Advance CBs after merge in rcutree_migrate_callbacks() Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 07/14] rcu/nocb: Reduce nocb_cb_wait() leaf rcu_node ->lock contention Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 08/14] rcu/nocb: Reduce __call_rcu_nocb_wake() " Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 09/14] rcu/nocb: Don't wake no-CBs GP kthread if timer posted under overload Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 10/14] rcu: Allow rcu_do_batch() to dynamically adjust batch sizes Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 11/14] EXP nohz: Add TICK_DEP_BIT_RCU Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 12/14] rcu/nohz: Force on tick when invoking lots of callbacks Paul E. McKenney
2019-08-02 15:15 ` [PATCH RFC tip/core/rcu 13/14] rcutorture: Force on tick for readers and callback flooders Paul E. McKenney
2019-08-02 15:15 ` [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs Paul E. McKenney
2019-08-04 14:43 ` Peter Zijlstra
2019-08-04 14:48 ` Peter Zijlstra
2019-08-04 18:41 ` Paul E. McKenney
2019-08-04 20:24 ` Paul E. McKenney
2019-08-05 4:19 ` Paul E. McKenney
2019-08-05 8:07 ` Peter Zijlstra
2019-08-05 14:47 ` Paul E. McKenney
2019-08-05 8:05 ` Peter Zijlstra
2019-08-05 14:54 ` Paul E. McKenney
2019-08-05 15:50 ` Peter Zijlstra
2019-08-05 17:48 ` Paul E. McKenney
2019-08-06 18:08 ` Paul E. McKenney
2019-08-07 21:41 ` Paul E. McKenney
2019-08-08 20:35 ` Paul E. McKenney
2019-08-08 21:30 ` Paul E. McKenney
2019-08-09 16:51 ` Paul E. McKenney
2019-08-09 18:07 ` Joel Fernandes
2019-08-09 18:39 ` Paul E. McKenney
2019-08-12 21:02 ` Frederic Weisbecker
2019-08-12 23:23 ` Paul E. McKenney
2019-08-13 1:33 ` Joel Fernandes
2019-08-13 12:30 ` Frederic Weisbecker
2019-08-13 14:48 ` Paul E. McKenney
2019-08-14 17:55 ` Joel Fernandes
2019-08-14 22:05 ` Paul E. McKenney
2019-08-15 15:07 ` Joel Fernandes
2019-08-15 17:23 ` Paul E. McKenney
2019-08-15 18:15 ` Joel Fernandes
2019-08-15 18:39 ` Paul E. McKenney
2019-08-15 19:42 ` Joel Fernandes
2019-08-13 21:06 ` Paul E. McKenney
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