From: Steven Rostedt <rostedt@goodmis.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com, 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, dhowells@redhat.com,
edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
joel@joelfernandes.org,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs before GP kthread is running
Date: Mon, 17 Feb 2020 15:25:17 -0500 [thread overview]
Message-ID: <20200217152517.26cc11ea@gandalf.local.home> (raw)
In-Reply-To: <20200215134208.GA9879@paulmck-ThinkPad-P72>
On Sat, 15 Feb 2020 05:42:08 -0800
"Paul E. McKenney" <paulmck@kernel.org> wrote:
>
> And does the following V2 look better?
>
For the issue I brought up, yes. But now I have to ask...
> @@ -1252,10 +1253,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
> */
> static void rcu_gp_kthread_wake(void)
> {
> - if ((current == rcu_state.gp_kthread &&
> - !in_irq() && !in_serving_softirq()) ||
> - !READ_ONCE(rcu_state.gp_flags) ||
> - !rcu_state.gp_kthread)
> + struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
> +
> + if ((current == t && !in_irq() && !in_serving_softirq()) ||
> + !READ_ONCE(rcu_state.gp_flags) || !t)
Why not test !t first? As that is the fastest operation in the if
statement, and will shortcut all the other operations if it is true.
As I like to micro-optimize ;-), for or (||) statements, I like to add
the fastest operations first. To me, that would be:
if (!t || READ_ONCE(rcu_state.gp_flags) ||
(current == t && !in_irq() && !in_serving_softirq()))
return;
Note, in_irq() reads preempt_count which is not always a fast operation.
-- Steve
> return;
> WRITE_ONCE(rcu_state.gp_wake_time, jiffies);
> WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq));
> @@ -3554,7 +3555,10 @@ static int __init rcu_spawn_gp_kthread(void)
> }
> rnp = rcu_get_root();
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
> - rcu_state.gp_kthread = t;
> + WRITE_ONCE(rcu_state.gp_activity, jiffies);
> + WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
> + // Reset .gp_activity and .gp_req_activity before setting .gp_kthread.
> + smp_store_release(&rcu_state.gp_kthread, t); /* ^^^ */
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> wake_up_process(t);
> rcu_spawn_nocb_kthreads();
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 488b71d..16ad7ad 100644
\
next prev parent reply other threads:[~2020-02-17 20:25 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-14 23:55 [PATCH tip/core/rcu 0/30] Miscellaneous fixes for v5.7 Paul E. McKenney
2020-02-14 23:55 ` [PATCH tip/core/rcu 01/30] nfs: Fix nfs_access_get_cached_rcu() sparse error paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 02/30] rcu: Warn on for_each_leaf_node_cpu_mask() from non-leaf paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 03/30] rcu: Fix exp_funnel_lock()/rcu_exp_wait_wake() datarace paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 04/30] rcu: Provide debug symbols and line numbers in KCSAN runs paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 05/30] rcu: Add WRITE_ONCE() to rcu_node ->qsmask update paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 06/30] rcu: Add WRITE_ONCE to rcu_node ->exp_seq_rq store paulmck
2020-02-15 3:47 ` Steven Rostedt
2020-02-15 10:58 ` Paul E. McKenney
2020-02-17 21:11 ` Joel Fernandes
2020-02-17 21:36 ` Paul E. McKenney
2020-02-14 23:55 ` [PATCH tip/core/rcu 07/30] rcu: Add READ_ONCE() to rcu_node ->gp_seq paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 08/30] rcu: Add WRITE_ONCE() to rcu_state ->gp_req_activity paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 09/30] rcu: Add WRITE_ONCE() to rcu_node ->qsmaskinitnext paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 10/30] locking/rtmutex: rcu: Add WRITE_ONCE() to rt_mutex ->owner paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 11/30] rcu: Add READ_ONCE() to rcu_segcblist ->tails[] paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 12/30] rcu: *_ONCE() for grace-period progress indicators paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 13/30] rcu: Fix typos in beginning comments paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 14/30] rcu: Add READ_ONCE() to rcu_data ->gpwrap paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 15/30] rcu: Add *_ONCE() to rcu_data ->rcu_forced_tick paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 16/30] rcu: Add *_ONCE() to rcu_node ->boost_kthread_status paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 17/30] timer: Use hlist_unhashed_lockless() in timer_pending() paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 18/30] rcu: Remove dead code from rcu_segcblist_insert_pend_cbs() paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 19/30] rcu: Add WRITE_ONCE() to rcu_state ->gp_start paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 20/30] rcu: Fix rcu_barrier_callback() race condition paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 21/30] rculist: Add brackets around cond argument in __list_check_rcu macro paulmck
2020-02-14 23:55 ` [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs before GP kthread is running paulmck
2020-02-15 3:53 ` Steven Rostedt
2020-02-15 11:01 ` Paul E. McKenney
2020-02-15 13:42 ` Paul E. McKenney
2020-02-17 20:25 ` Steven Rostedt [this message]
2020-02-17 22:03 ` Paul E. McKenney
2020-02-17 22:21 ` Steven Rostedt
2020-02-17 23:03 ` Paul E. McKenney
2020-02-14 23:56 ` [PATCH tip/core/rcu 23/30] rcu: Add missing annotation for rcu_nocb_bypass_lock() paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 24/30] rcu/nocb: Add missing annotation for rcu_nocb_bypass_unlock() paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 25/30] rcu: Optimize and protect atomic_cmpxchg() loop paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 26/30] rcu: Tighten rcu_lockdep_assert_cblist_protected() check paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 27/30] rcu: Make nocb_gp_wait() double-check unexpected-callback warning paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 28/30] rcu: Mark rcu_state.ncpus to detect concurrent writes paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 29/30] rcu: Mark rcu_state.gp_seq " paulmck
2020-02-14 23:56 ` [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs paulmck
2020-02-25 10:24 ` Boqun Feng
2020-02-26 3:14 ` Paul E. McKenney
2020-02-26 4:18 ` Paul E. McKenney
2020-02-26 6:14 ` Boqun Feng
2020-02-26 15:02 ` Paul E. McKenney
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