On Thu, 28 Apr 2016 14:57:24 +0200 Daniel Wagner <wagi@xxxxxxxxx> wrote: > From: Daniel Wagner <daniel.wagner@xxxxxxxxxxxx> > > 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@handman 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@handman 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@xxxxxxxxxxxx> > --- > > 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; -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
