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Date:	Mon, 16 May 2016 16:33:00 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Daniel Wagner <wagi@...om.org>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Daniel Wagner <daniel.wagner@...-carit.de>
Subject: Re: [PATCH v2] sched/completion: convert completions to use simple
 wait  queues

On Thu, 28 Apr 2016 14:57:24 +0200
Daniel Wagner <wagi@...om.org> wrote:

> From: Daniel Wagner <daniel.wagner@...-carit.de>
> 
> Completions have no long lasting callbacks and therefore do not need
> the complex waitqueue variant.  Use simple waitqueues which reduces
> the contention on the waitqueue lock.
> 
> This was a carry forward from v3.10-rt, with some RT specific chunks,
> dropped, and updated to align with names that were chosen to match the
> simple waitqueue support.
> 
> While the conversion of complete() is trivial the complete_all() is
> more difficult. complete_all() could be called from IRQ context and
> therefore we don't want to wake up potentially a lot of
> waiters. Therefore, only the first waiter is waked and the rest of the
> waiters are waked by the first waiter. To avoid a larger struct
> completion data structure the done integer is spitted into a unsigned
> short for the flags and one unsigned short done.
> 
> The size of vmlinuz doesn't change too much:
> 
> 
> add/remove: 3/0 grow/shrink: 3/10 up/down: 242/-236 (6)
> function                                     old     new   delta
> swake_up_all_locked                            -     181    +181
> __kstrtab_swake_up_all_locked                  -      20     +20
> __ksymtab_swake_up_all_locked                  -      16     +16
> complete_all                                  73      87     +14
> try_wait_for_completion                       99     107      +8
> completion_done                               40      43      +3
> complete                                      73      65      -8
> wait_for_completion_timeout                  283     265     -18
> wait_for_completion_killable_timeout         319     301     -18
> wait_for_completion_io_timeout               283     265     -18
> wait_for_completion_io                       275     257     -18
> wait_for_completion                          275     257     -18
> wait_for_completion_interruptible_timeout     304     285     -19
> kexec_purgatory                            26473   26449     -24
> wait_for_completion_killable                 544     499     -45
> wait_for_completion_interruptible            522     472     -50
> 
> 
> The downside of this approach is we can only wake up 32k waiters
> instead of 2m. Though this doesn't seem to be a real issue.
> 
> With a lockdep inspired waiter tracker I verified how many waiters
> are queued up on a complete() or complete_all() call.
> 
> The first line contains starts with class name of the swait object
> followed by 4 columns which count the number of waiters. After that
> there is a left ip/symbol column for the waiter and the right
> ip/symbol column for the waker.
> 
> I run mmtest with config/config-global-dhp__scheduler-unbound with
> additional kernbench:
> 
> 
> swait_stat version 0.1
> ---------------------------------------------------------------------------------------------
>                               class name     1 waiter    2 waiters    3 waiters   4+ waiters
> ---------------------------------------------------------------------------------------------
>                              &rsp->gp_wq       129572            0            0            0
>          [<ffffffff810c5b81>] kthread+0x101/0x120
>                                                 20154          [<ffffffff8110cf1f>] rcu_gp_kthread_wake+0x3f/0x50
>                                                   535          [<ffffffff8110f603>] rcu_nocb_kthread+0x423/0x4b0
>                                                 43867          [<ffffffff8110cfc1>] rcu_report_qs_rsp+0x51/0x80
>                                                 44010          [<ffffffff8110d105>] rcu_report_qs_rnp+0x115/0x130
>                                                 15882          [<ffffffff81111778>] rcu_process_callbacks+0x268/0x4a0
>                                                  4437          [<ffffffff8111043c>] note_gp_changes+0xbc/0xc0
>                                                   687          [<ffffffff8110f83e>] rcu_eqs_enter_common+0x1ae/0x1e0
>                              &x->wait#11        39002            0            0            0
>          [<ffffffff810a4c43>] _do_fork+0x253/0x3c0
>                                                 39002          [<ffffffff810a2e9b>] mm_release+0xbb/0x140
>                      &rnp->nocb_gp_wq[1]        10277            0            0            0
>          [<ffffffff810c5b81>] kthread+0x101/0x120
>                                                 10277          [<ffffffff810c5b81>] kthread+0x101/0x120
>                            &rdp->nocb_wq         9862            0            0            0
>          [<ffffffff810c5b81>] kthread+0x101/0x120
>                                                  4931          [<ffffffff8110ce05>] wake_nocb_leader+0x45/0x50
>                                                  4290          [<ffffffff8110ced7>] __call_rcu_nocb_enqueue+0xc7/0xd0
>                                                   629          [<ffffffff8110f728>] rcu_eqs_enter_common+0x98/0x1e0
>                                                    12          [<ffffffff811115e5>] rcu_process_callbacks+0xd5/0x4a0
>                      &rnp->nocb_gp_wq[0]         9769            0            0            0
>          [<ffffffff810c5b81>] kthread+0x101/0x120
>                                                  9769          [<ffffffff810c5b81>] kthread+0x101/0x120
>                               &x->wait#8         4123            0            0            0
>          [<ffffffffa011f03f>] xfs_buf_submit_wait+0x7f/0x280 [xfs]
>                                                  4123          [<ffffffffa011e855>] xfs_buf_ioend+0xf5/0x230 [xfs]
>                           (wait).wait#98         1594            0            0            0
>          [<ffffffff81471e94>] blk_execute_rq+0xb4/0x130
>                                                  1594          [<ffffffff81471f33>] blk_end_sync_rq+0x23/0x30
>                                 &x->wait          827            0            0            0
>          [<ffffffff810c571d>] kthread_park+0x4d/0x60
>          [<ffffffff810c602f>] kthread_stop+0x4f/0x140
>                                                   320          [<ffffffff810c566c>] __kthread_parkme+0x3c/0x70
>                                                   507          [<ffffffff810a2e9b>] mm_release+0xbb/0x140
>                          (done).wait#119          512            0            0            0
>          [<ffffffff810c5836>] kthread_create_on_node+0x106/0x1d0
>                                                   512          [<ffffffff810c5b51>] kthread+0xd1/0x120
>                               &x->wait#5          347            0            0            0
>          [<ffffffff810beb97>] flush_work+0x127/0x1d0
>          [<ffffffff810bc976>] flush_workqueue+0x176/0x5b0
>                                                   273          [<ffffffff810bc742>] wq_barrier_func+0x12/0x20
>                                                    74          [<ffffffff810bf308>] pwq_dec_nr_in_flight+0x98/0xa0
>                           (done).wait#10          315            0            0            0
>          [<ffffffff810c5836>] kthread_create_on_node+0x106/0x1d0
>                                                   315          [<ffffffff810c5b51>] kthread+0xd1/0x120
>                               &x->wait#4          298            0            0            0
>          [<ffffffff815d7f2b>] devtmpfs_create_node+0x10b/0x150
>                                                   298          [<ffffffff815d7dce>] devtmpfsd+0x10e/0x160
>                               &x->wait#3          171            0            0            0
>          [<ffffffff8110bd26>] __wait_rcu_gp+0xc6/0xf0
>                                                   171          [<ffffffff8110bc52>] wakeme_after_rcu+0x12/0x20
> [...]
> 
> The stats show that at least for this workload there was never more
> than 1 waiter when complete() or complete_all() was called. That
> matches also the code review of all complete_all() calls.
> 
> One common pattern is
> 
>  - prepare packet to transmit
>  - complete_init(&done)
>  - trigger hardware to transmit packet
>  - wait_for_completion(&done)
>  - irq handler calls complete_all(&done)
> 
> e.g. see drivers/i2c/busses/i2c-bcm-iproc.c
> git
> The filesystem system uses completion in a more complex pattern which
> I couldn't really decipher but some simple fs benchmarks didn't show
> multiple waiters.
> 
> Only one complete_all() user could been identified so far, which happens
> to be drivers/base/power/main.c. Several waiters appear when suspend
> to disk or mem is executed.
> 
> As one can see above in the swait_stat output, the fork() path is
> using completion. A histogram of a fork bomp (1000 forks) benchmark
> shows a slight performance drop by 4%.
> 
> [wagi@...dman completion-test-5 (master)]$ cat forky-4.6.0-rc4.txt | perl histo -min 0.12 -max 0.20 -int 0.01 -stars -scale 10
> # NumSamples = 1000; Max = 0.208; Min = 0.123
> # Mean = 0.146406; Variance = 0.000275351163999956; SD = 0.0165937085668019
> # Each * represents a count of 10
>      0.1200 - 0.1300 [   113]: ************
>      0.1300 - 0.1400 [   324]: *********************************
>      0.1400 - 0.1500 [   219]: **********************
>      0.1500 - 0.1600 [   139]: **************
>      0.1600 - 0.1700 [    94]: **********
>      0.1700 - 0.1800 [    54]: ******
>      0.1800 - 0.1900 [    37]: ****
>      0.1900 - 0.2000 [    18]: **
> 
> [wagi@...dman completion-test-5 (master)]$ cat forky-4.6.0-rc4-00001-g0a16067.txt | perl histo -min 0.12 -max 0.20 -int 0.01 -stars -scale 10
> # NumSamples = 1000; Max = 0.207; Min = 0.121
> # Mean = 0.152056; Variance = 0.000295474863999994; SD = 0.0171893823042014
> # Each * represents a count of 10
>      0.1200 - 0.1300 [    17]: **
>      0.1300 - 0.1400 [   282]: *****************************
>      0.1400 - 0.1500 [   240]: ************************
>      0.1500 - 0.1600 [   158]: ****************
>      0.1600 - 0.1700 [   114]: ************
>      0.1700 - 0.1800 [    94]: **********
>      0.1800 - 0.1900 [    66]: *******
>      0.1900 - 0.2000 [    25]: ***
>      0.2000 - 0.2100 [     1]: *
> 
> Compiling a kernel 100 times results in following statistics gather
> by 'time make -j200'
> 
> user
>                                         mean                std                var                max                min
>                kernbech-4.6.0-rc4      9.126             0.2919            0.08523               9.92               8.55
>    kernbech-4.6.0-rc4-00001-g0...       9.24  -1.25%     0.2768   5.17%    0.07664  10.07%      10.11  -1.92%       8.44   1.29%
> 
> 
> system
>                                         mean                std                var                max                min
>                kernbech-4.6.0-rc4  1.676e+03              2.409              5.804          1.681e+03          1.666e+03
>    kernbech-4.6.0-rc4-00001-g0...  1.675e+03   0.07%      2.433  -1.01%      5.922  -2.03%  1.682e+03  -0.03%   1.67e+03  -0.20%
> 
> 
> elapsed
>                                         mean                std                var                max                min
>                kernbech-4.6.0-rc4  2.303e+03              26.67              711.1          2.357e+03          2.232e+03
>    kernbech-4.6.0-rc4-00001-g0...  2.298e+03   0.23%      28.75  -7.83%      826.8 -16.26%  2.348e+03   0.38%  2.221e+03   0.49%
> 
> 
> CPU
>                                         mean                std                var                max                min
>                kernbech-4.6.0-rc4  4.418e+03               48.9          2.391e+03          4.565e+03          4.347e+03
>    kernbech-4.6.0-rc4-00001-g0...  4.424e+03  -0.15%      55.73 -13.98%  3.106e+03 -29.90%  4.572e+03  -0.15%  4.356e+03  -0.21%
> 
> 
> While the mean is slightly less the var and std are increasing quite
> noticeable.
> 
> Signed-off-by: Daniel Wagner <daniel.wagner@...-carit.de>
> ---
> 
> I have also created a picture with the histograms for the above
> tests. Since most of use are not able to process the postscript data
> directly I omitted it to attach it directly. You can find it
> here:
> 
> http://monom.org/data/completion/kernbench-completion-swait.png
> 
> changes since v1: none, just more tests and bigger commit message.
> 
> 
>  include/linux/completion.h | 23 ++++++++++++++++-------
>  include/linux/swait.h      |  1 +
>  kernel/sched/completion.c  | 43 ++++++++++++++++++++++++++-----------------
>  kernel/sched/swait.c       | 24 ++++++++++++++++++++++++
>  4 files changed, 67 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/completion.h b/include/linux/completion.h
> index 5d5aaae..45fd91a 100644
> --- a/include/linux/completion.h
> +++ b/include/linux/completion.h
> @@ -8,7 +8,7 @@
>   * See kernel/sched/completion.c for details.
>   */
>  
> -#include <linux/wait.h>
> +#include <linux/swait.h>
>  
>  /*
>   * struct completion - structure used to maintain state for a "completion"
> @@ -22,13 +22,22 @@
>   * reinit_completion(), and macros DECLARE_COMPLETION(),
>   * DECLARE_COMPLETION_ONSTACK().
>   */
> +
> +#define COMPLETION_DEFER (1 << 0)
> +
>  struct completion {
> -	unsigned int done;
> -	wait_queue_head_t wait;
> +	union {
> +		struct {
> +			unsigned short flags;
> +			unsigned short done;
> +		};
> +		unsigned int val;
> +	};
> +	struct swait_queue_head wait;
>  };
>  
>  #define COMPLETION_INITIALIZER(work) \
> -	{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
> +	{ 0, 0, __SWAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
>  
>  #define COMPLETION_INITIALIZER_ONSTACK(work) \
>  	({ init_completion(&work); work; })
> @@ -72,8 +81,8 @@ struct completion {
>   */
>  static inline void init_completion(struct completion *x)
>  {
> -	x->done = 0;
> -	init_waitqueue_head(&x->wait);
> +	x->val = 0;
> +	init_swait_queue_head(&x->wait);
>  }
>  
>  /**
> @@ -85,7 +94,7 @@ static inline void init_completion(struct completion *x)
>   */
>  static inline void reinit_completion(struct completion *x)
>  {
> -	x->done = 0;
> +	x->val = 0;
>  }
>  
>  extern void wait_for_completion(struct completion *);
> diff --git a/include/linux/swait.h b/include/linux/swait.h
> index c1f9c62..83f004a 100644
> --- a/include/linux/swait.h
> +++ b/include/linux/swait.h
> @@ -87,6 +87,7 @@ static inline int swait_active(struct swait_queue_head *q)
>  extern void swake_up(struct swait_queue_head *q);
>  extern void swake_up_all(struct swait_queue_head *q);
>  extern void swake_up_locked(struct swait_queue_head *q);
> +extern void swake_up_all_locked(struct swait_queue_head *q);
>  
>  extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);
>  extern void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state);
> diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
> index 8d0f35d..d4dccd3 100644
> --- a/kernel/sched/completion.c
> +++ b/kernel/sched/completion.c
> @@ -30,10 +30,10 @@ void complete(struct completion *x)
>  {
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&x->wait.lock, flags);
> +	raw_spin_lock_irqsave(&x->wait.lock, flags);
>  	x->done++;
> -	__wake_up_locked(&x->wait, TASK_NORMAL, 1);
> -	spin_unlock_irqrestore(&x->wait.lock, flags);
> +	swake_up_locked(&x->wait);
> +	raw_spin_unlock_irqrestore(&x->wait.lock, flags);
>  }
>  EXPORT_SYMBOL(complete);
>  
> @@ -50,10 +50,15 @@ void complete_all(struct completion *x)
>  {
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&x->wait.lock, flags);
> -	x->done += UINT_MAX/2;
> -	__wake_up_locked(&x->wait, TASK_NORMAL, 0);
> -	spin_unlock_irqrestore(&x->wait.lock, flags);
> +	raw_spin_lock_irqsave(&x->wait.lock, flags);
> +	x->done += USHRT_MAX/2;
> +	if (irqs_disabled_flags(flags)) {
> +		x->flags = COMPLETION_DEFER;
> +		swake_up_locked(&x->wait);

Does it impact performance if we always did this? This would allow
us to drop the special case and the changes to struct compaction.

> +	} else {
> +		swake_up_all_locked(&x->wait);
> +	}
> +	raw_spin_unlock_irqrestore(&x->wait.lock, flags);
>  }
>  EXPORT_SYMBOL(complete_all);
>  
> @@ -62,20 +67,20 @@ do_wait_for_common(struct completion *x,
>  		   long (*action)(long), long timeout, int state)
>  {
>  	if (!x->done) {
> -		DECLARE_WAITQUEUE(wait, current);
> +		DECLARE_SWAITQUEUE(wait);
>  
> -		__add_wait_queue_tail_exclusive(&x->wait, &wait);
> +		__prepare_to_swait(&x->wait, &wait);
>  		do {
>  			if (signal_pending_state(state, current)) {
>  				timeout = -ERESTARTSYS;
>  				break;
>  			}
>  			__set_current_state(state);
> -			spin_unlock_irq(&x->wait.lock);
> +			raw_spin_unlock_irq(&x->wait.lock);
>  			timeout = action(timeout);
> -			spin_lock_irq(&x->wait.lock);
> +			raw_spin_lock_irq(&x->wait.lock);
>  		} while (!x->done && timeout);
> -		__remove_wait_queue(&x->wait, &wait);
> +		__finish_swait(&x->wait, &wait);
>  		if (!x->done)
>  			return timeout;
>  	}
> @@ -89,9 +94,13 @@ __wait_for_common(struct completion *x,
>  {
>  	might_sleep();
>  
> -	spin_lock_irq(&x->wait.lock);
> +	raw_spin_lock_irq(&x->wait.lock);
>  	timeout = do_wait_for_common(x, action, timeout, state);
> -	spin_unlock_irq(&x->wait.lock);
> +	raw_spin_unlock_irq(&x->wait.lock);
> +	if (x->flags & COMPLETION_DEFER) {
> +		x->flags = 0;
> +		swake_up_all(&x->wait);
> +	}
>  	return timeout;
>  }
>  
> @@ -277,12 +286,12 @@ bool try_wait_for_completion(struct completion *x)
>  	if (!READ_ONCE(x->done))
>  		return 0;
>  
> -	spin_lock_irqsave(&x->wait.lock, flags);
> +	raw_spin_lock_irqsave(&x->wait.lock, flags);
>  	if (!x->done)
>  		ret = 0;
>  	else
>  		x->done--;
> -	spin_unlock_irqrestore(&x->wait.lock, flags);
> +	raw_spin_unlock_irqrestore(&x->wait.lock, flags);
>  	return ret;
>  }
>  EXPORT_SYMBOL(try_wait_for_completion);
> @@ -311,7 +320,7 @@ bool completion_done(struct completion *x)
>  	 * after it's acquired the lock.
>  	 */
>  	smp_rmb();
> -	spin_unlock_wait(&x->wait.lock);
> +	raw_spin_unlock_wait(&x->wait.lock);
>  	return true;
>  }
>  EXPORT_SYMBOL(completion_done);
> diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
> index 82f0dff..efe366b 100644
> --- a/kernel/sched/swait.c
> +++ b/kernel/sched/swait.c
> @@ -72,6 +72,30 @@ void swake_up_all(struct swait_queue_head *q)
>  }
>  EXPORT_SYMBOL(swake_up_all);
>  
> +void swake_up_all_locked(struct swait_queue_head *q)
> +{
> +	struct swait_queue *curr;
> +	LIST_HEAD(tmp);
> +
> +	if (!swait_active(q))
> +		return;
> +
> +	list_splice_init(&q->task_list, &tmp);
> +	while (!list_empty(&tmp)) {
> +		curr = list_first_entry(&tmp, typeof(*curr), task_list);
> +
> +		wake_up_state(curr->task, TASK_NORMAL);
> +		list_del_init(&curr->task_list);
> +
> +		if (list_empty(&tmp))
> +			break;
> +
> +		raw_spin_unlock_irq(&q->lock);
> +		raw_spin_lock_irq(&q->lock);
> +	}
> +}
> +EXPORT_SYMBOL(swake_up_all_locked);
> +
>  void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait)
>  {
>  	wait->task = current;

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