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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Steven Rostedt <rostedt@goodmis.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 15:03:03 -0800	[thread overview]
Message-ID: <20200217230303.GZ2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200217172131.1f4c48d2@gandalf.local.home>

On Mon, Feb 17, 2020 at 05:21:31PM -0500, Steven Rostedt wrote:
> On Mon, 17 Feb 2020 14:03:56 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> 
> > And what is a day without micro-optimization of a slowpath?  :-)
> 
> A day you have off, but still find yourself working ;-)
> 
> > 
> > OK, let's see...
> > 
> > Grace-period kthread wakeups are normally mediated by rcu_start_this_gp(),
> > which uses a funnel lock to consolidate concurrent requests to start
> > a grace period.  If a grace period is already in progress, it refrains
> > from doing a wakeup because that means that the grace-period kthread
> > will check for another grace period being needed at the end of the
> > current grace period.
> > 
> > Exceptions include:
> > 
> > o	The wakeup reporting the last quiescent state of the current
> > 	grace period.
> > 
> > o	Emergency situations such as callback overloads and RCU CPU stalls.
> > 
> > So on a busy system that is not overloaded, the common case is that
> > rcu_gp_kthread_wake() is invoked only once per grace period because there
> > is no emergency and there is a grace period in progress.  If this system
> > has short idle periods and a fair number of quiescent states, a reasonable
> > amount of idle time, then the last quiescent state will not normally be
> > detected by the grace-period kthread.  But workloads can of course vary.
> > 
> > The "!t" holds only during early boot.  So we could put a likely() around
> > the "t".  But more to the point, at runtime, "!t" would always be false,
> > so it really should be last in the list of "||" clauses.  This isn't
> > enough of a fastpath for a static branch to make sense.
> 
> Hey! Does that mean we can add a static branch for that check?
> 
> 
> struct static_key rcu_booting = STATIC_KEY_INIT_TRUE;
> 
> [...]
> 
> 	if (READ_ONCE(rcu_state.gp_flags) ||
> 	    (current == t && !in_irq() && !in_serving_softirq())
> 		return;
> 
> 	if (static_branch_unlikely(&rcu_booting) && !t)
> 		return;
> 
> At end of boot:
> 
> 	static_key_disable(&rcu_booting);
> 
> That way we can really micro-optimize the slow path, and it basically
> becomes a nop!

;-) ;-) ;-)

There might be some place where static branches would be of use in RCU,
but unfortunately none that I can think of on the read side.  Though I
am once again making progress on Lai Jiangshan's patchset, which might
one day result in inlining of __rcu_read_lock() and __rcu_read_unlock()
in PREEMPT=y kernels.

							Thanx, Paul

> -- Steve
> 
> > 
> > The "!READ_ONCE(rcu_state.gp_flags)" will normally hold, though it is
> > false often enough to pay for itself.  Or has been in the past, anyway.
> > I suspect that access to the global variable rcu_state.gp_flags is not
> > always fast either.
> > 
> > So I am having difficulty talking myself into modifying this one given
> > the frequency of operations.
> > 
> > 							Thanx, Paul
> 

  reply	other threads:[~2020-02-17 23:03 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
2020-02-17 22:03           ` Paul E. McKenney
2020-02-17 22:21             ` Steven Rostedt
2020-02-17 23:03               ` Paul E. McKenney [this message]
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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