Date: Wed, 07 Nov 2012 10:22:01 +0800 From: David Xu <davidxu@freebsd.org> To: attilio@freebsd.org Cc: freebsd-hackers <freebsd-hackers@freebsd.org>, Jeff Roberson <jeff@freebsd.org>, Andriy Gapon <avg@freebsd.org> Subject: Re: ule+smp: small optimization for turnstile priority lending Message-ID: <5099C5C9.4040703@freebsd.org> In-Reply-To: <CAJ-FndBGEgkrKJ9bNdq0QrdyYZb=LXUsAG3wz5Lp-HLUBd5d9w@mail.gmail.com> References: <50587F8D.9060102@FreeBSD.org> <505AD2A5.6060008@freebsd.org> <CAJ-FndBGEgkrKJ9bNdq0QrdyYZb=LXUsAG3wz5Lp-HLUBd5d9w@mail.gmail.com>
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On 2012/11/06 19:03, Attilio Rao wrote:
> On 9/20/12, David Xu <davidxu@freebsd.org> wrote:
>> On 2012/09/18 22:05, Andriy Gapon wrote:
>>>
>>> Here is a snippet that demonstrates the issue on a supposedly fully
>>> loaded
>>> 2-processor system:
>>>
>>> 136794 0 3670427870244462 KTRGRAPH group:"thread", id:"Xorg tid
>>> 102818",
>>> state:"running", attributes: prio:122
>>>
>>> 136793 0 3670427870241000 KTRGRAPH group:"thread", id:"cc1plus tid
>>> 111916",
>>> state:"yielding", attributes: prio:183, wmesg:"(null)", lockname:"(null)"
>>>
>>> 136792 1 3670427870240829 KTRGRAPH group:"thread", id:"idle: cpu1 tid
>>> 100004",
>>> state:"running", attributes: prio:255
>>>
>>> 136791 1 3670427870239520 KTRGRAPH group:"load", id:"CPU 1 load",
>>> counter:0,
>>> attributes: none
>>>
>>> 136790 1 3670427870239248 KTRGRAPH group:"thread", id:"firefox tid
>>> 113473",
>>> state:"blocked", attributes: prio:122, wmesg:"(null)", lockname:"unp_mtx"
>>>
>>> 136789 1 3670427870237697 KTRGRAPH group:"load", id:"CPU 0 load",
>>> counter:2,
>>> attributes: none
>>>
>>> 136788 1 3670427870236394 KTRGRAPH group:"thread", id:"firefox tid
>>> 113473",
>>> point:"wokeup", attributes: linkedto:"Xorg tid 102818"
>>>
>>> 136787 1 3670427870236145 KTRGRAPH group:"thread", id:"Xorg tid
>>> 102818",
>>> state:"runq add", attributes: prio:122, linkedto:"firefox tid 113473"
>>>
>>> 136786 1 3670427870235981 KTRGRAPH group:"load", id:"CPU 1 load",
>>> counter:1,
>>> attributes: none
>>>
>>> 136785 1 3670427870235707 KTRGRAPH group:"thread", id:"Xorg tid
>>> 102818",
>>> state:"runq rem", attributes: prio:176
>>>
>>> 136784 1 3670427870235423 KTRGRAPH group:"thread", id:"Xorg tid
>>> 102818",
>>> point:"prio", attributes: prio:176, new prio:122, linkedto:"firefox tid
>>> 113473"
>>>
>>> 136783 1 3670427870202392 KTRGRAPH group:"thread", id:"firefox tid
>>> 113473",
>>> state:"running", attributes: prio:104
>>>
>>> See how how the Xorg thread was forced from CPU 1 to CPU 0 where it
>>> preempted
>>> cc1plus thread (I do have preemption enabled) only to leave CPU 1 with
>>> zero load.
>>>
>>> Here is a proposed solution:
>>>
>>> turnstile_wait: optimize priority lending to a thread on a runqueue
>>>
>>> As the current thread is definitely going into mi_switch, it now
>>> removes
>>> its load before doing priority propagation which can potentially
>>> result
>>> in sched_add. In the SMP && ULE case the latter searches for the
>>> least loaded CPU to place a boosted thread, which is supposedly
>>> about
>>> to run.
>>>
>>> diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c
>>> index 8e466cd..3299cae 100644
>>> --- a/sys/kern/sched_ule.c
>>> +++ b/sys/kern/sched_ule.c
>>> @@ -1878,7 +1878,10 @@ sched_switch(struct thread *td, struct thread
>>> *newtd, int
>>> flags)
>>> /* This thread must be going to sleep. */
>>> TDQ_LOCK(tdq);
>>> mtx = thread_lock_block(td);
>>> - tdq_load_rem(tdq, td);
>>> +#if defined(SMP)
>>> + if ((flags & SW_TYPE_MASK) != SWT_TURNSTILE)
>>> +#endif
>>> + tdq_load_rem(tdq, td);
>>> }
>>> /*
>>> * We enter here with the thread blocked and assigned to the
>>> @@ -2412,6 +2415,21 @@ sched_rem(struct thread *td)
>>> tdq_setlowpri(tdq, NULL);
>>> }
>>>
>>> +void
>>> +sched_load_rem(struct thread *td)
>>> +{
>>> + struct tdq *tdq;
>>> +
>>> + KASSERT(td == curthread,
>>> + ("sched_rem_load: only curthread is supported"));
>>> + KASSERT(td->td_oncpu == td->td_sched->ts_cpu,
>>> + ("thread running on cpu different from ts_cpu"));
>>> + tdq = TDQ_CPU(td->td_sched->ts_cpu);
>>> + TDQ_LOCK_ASSERT(tdq, MA_OWNED);
>>> + MPASS(td->td_lock == TDQ_LOCKPTR(tdq));
>>> + tdq_load_rem(tdq, td);
>>> +}
>>> +
>>> /*
>>> * Fetch cpu utilization information. Updates on demand.
>>> */
>>> diff --git a/sys/kern/subr_turnstile.c b/sys/kern/subr_turnstile.c
>>> index 31d16fe..d1d68e9 100644
>>> --- a/sys/kern/subr_turnstile.c
>>> +++ b/sys/kern/subr_turnstile.c
>>> @@ -731,6 +731,13 @@ turnstile_wait(struct turnstile *ts, struct thread
>>> *owner,
>>> int queue)
>>> LIST_INSERT_HEAD(&ts->ts_free, td->td_turnstile, ts_hash);
>>> }
>>> thread_lock(td);
>>> +#if defined(SCHED_ULE) && defined(SMP)
>>> + /*
>>> + * Remove load earlier so that it does not affect cpu selection
>>> + * for a thread waken up due to priority lending, if any.
>>> + */
>>> + sched_load_rem(td);
>>> +#endif
>>> thread_lock_set(td, &ts->ts_lock);
>>> td->td_turnstile = NULL;
>>>
>>> diff --git a/sys/sys/sched.h b/sys/sys/sched.h
>>> index 4b8387c..b1ead1b 100644
>>> --- a/sys/sys/sched.h
>>> +++ b/sys/sys/sched.h
>>> @@ -110,6 +110,9 @@ void sched_preempt(struct thread *td);
>>> void sched_add(struct thread *td, int flags);
>>> void sched_clock(struct thread *td);
>>> void sched_rem(struct thread *td);
>>> +#if defined(SCHED_ULE) && defined(SMP)
>>> +void sched_load_rem(struct thread *td);
>>> +#endif
>>> void sched_tick(int cnt);
>>> void sched_relinquish(struct thread *td);
>>> struct thread *sched_choose(void);
>>>
>>
>> I found another scenario in taskqueue, in the function
>> taskqueue_terminate, current thread tries to wake
>> another thread up and sleep immediately, the tq_mutex sometimes
>> is a spinlock. So if you remove one thread load from current cpu
>> before wakeup, the resumed thread may be put on same cpu,
>> so it will optimize the cpu scheduling too.
>
> I think that in order to fit with sched_add() modifies I have in mind
> (see other patches within this thread) wakeup() should grow a new
> argument, or maybe we can use wakeup_flags() new KPI.
> If the latter is the case, I would also propose to let wakeup_one() to
> be absorbed into wakeup_flags() with its own flag.
>
Yes, I like the idea.
> Attilio
>
>
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