Messages in this thread | | | Date | Tue, 22 Apr 2014 20:56:11 +0900 | From | Dongsheng Yang <> | Subject | Re: [PATCH 4/8] sched/core: Skip wakeup when task is already running. |
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On 04/16/2014 07:22 PM, Dongsheng Yang wrote: > On 04/15/2014 10:53 PM, Peter Zijlstra wrote: >> On Tue, Apr 15, 2014 at 09:32:53PM +0900, Dongsheng Yang wrote: >> >> How can you get there with ->state == RUNNING? try_to_wake_up*() bail >> when !(->state & state). > Yes, try_to_wake_up() did this check. But other callers would miss it. > > With the following code ,I can get the actual message of waking up > a running task > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 9f63275..1369cae 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1418,8 +1418,10 @@ static void ttwu_activate(struct rq *rq, struct > task_stru > static void > ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags) > { > - if (p->state == TASK_RUNNING) > + if (p->state == TASK_RUNNING) { > + printk("Wakeup a running task."); > return; > + } > > check_preempt_curr(rq, p, wake_flags); > trace_sched_wakeup(p, true); > > > # grep "Wakeup" /var/log/messages > Apr 15 20:16:21 localhost kernel: [ 5.436505] Wakeup a running task. > Apr 15 20:16:21 localhost kernel: [ 7.776042] Wakeup a running task. > Apr 15 20:16:21 localhost kernel: [ 9.324274] Wakeup a running task.
Hi Peter, after some more investigation, I think I got the problem, which is that some other task set p->state to TASK_RUNNING without holding p->pi_lock.
Scenario as attached graph shown, if some other task set p->state to TASK_RUNNING after the check if (! (p->state & state)), then we are wasting time to wake up a running task in try_to_wake_up().
If the analyse is right, I think there are two methods to solve this problem: * Skip in ttwu_do_wakeup() when p->state is running, as what my patch did. * Add a locking when we set p->state, lots of work to do and I am afraid it will hurt the performance of kernel.
The following message is the backtrace info I got when it happened:
(gdb) bt #0 try_to_wake_up (p=0xffff88027e651930, state=1, wake_flags=0) at kernel/sched/core.c:1605 #1 0xffffffff81099532 in default_wake_function (curr=<value optimized out>, mode=<value optimized out>, wake_flags=<value optimized out>, key=<value optimized out>) at kernel/sched/core.c:2853 #2 0xffffffff810aa489 in __wake_up_common (q=0xffff88027f03f210, mode=1, nr_exclusive=1, wake_flags=0, key=0x4) at kernel/sched/wait.c:75 #3 0xffffffff810aa838 in __wake_up (q=0xffff88027f03f210, mode=1, nr_exclusive=1, key=0x4) at kernel/sched/wait.c:97 #4 0xffffffff813cd0a4 in n_tty_check_unthrottle (tty=0xffff88027f03ec00, file=0xffff880278ab1b00, buf=0x7fff0fcf9720 "\r\nyum install -y ./a/alsa-plugins-pulseaudio-1.0.27-1.fc19.x86_64.rpml -y ./a/alsa-lib-1.0.27.1-2.fc19.x86_64.rpm.noarch.rpm\r\nyum install -y ./a/albatross-xfwm4-theme-1.2-5.fc19.noarch.rpm\r\nyum instal"..., nr=16315) at drivers/tty/n_tty.c:280 #5 n_tty_read (tty=0xffff88027f03ec00, file=0xffff880278ab1b00, buf=0x7fff0fcf9720 "\r\nyum install -y ./a/alsa-plugins-pulseaudio-1.0.27-1.fc19.x86_64.rpml -y ./a/alsa-lib-1.0.27.1-2.fc19.x86_64.rpm.noarch.rpm\r\nyum install -y ./a/albatross-xfwm4-theme-1.2-5.fc19.noarch.rpm\r\nyum instal"..., nr=16315) at drivers/tty/n_tty.c:2259 #6 0xffffffff813c5667 in tty_read (file=0xffff880278ab1b00, buf=0x7fff0fcf9720 "\r\nyum install -y ./a/alsa-plugins-pulseaudio-1.0.27-1.fc19.x86_64.rpml -y ./a/alsa-lib-1.0.27.1-2.fc19.x86_64.rpm.noarch.rpm\r\nyum in---Type <return> to continue, or q <return> to quit---q Quit (gdb) p p->state ------> Currently, p->state is TASK_RUNNING. $1 = 0 (gdb) l 1600 1601 success = 1; /* we're going to change ->state */ 1602 cpu = task_cpu(p); 1603 1604 if (p->state == TASK_RUNNING) { 1605 printk("Wake up a running task."); 1606 } 1607 if (p->on_rq && ttwu_remote(p, wake_flags)) 1608 goto stat; 1609 (gdb) > > So, I think there are some caller of ttwu_do_wakeup() is attempt to wake > up a running task. >> . >> > > -- > To unsubscribe from this list: send the line "unsubscribe > linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > . >
CPU1 CPU2 raw_spin_lock_irqsave(&p->pi_lock, flags); set_task_state(tsk, TASK_INTERRUPTIBLE); if (!(p->state & state)) ---> TASK_INTERRUPTIBLE | goto out; | cpu = task_cpu(p); set_task_state(tsk, TASK_RUNNING); ... ---> TASK_RUNNING
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