Date: Tue, 6 Nov 2012 20:17:20 -1000 (HST) From: Jeff Roberson <jroberson@jroberson.net> To: David Xu <davidxu@freebsd.org> Cc: attilio@freebsd.org, 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: <alpine.BSF.2.00.1211062016380.4081@desktop> In-Reply-To: <5099C5C9.4040703@freebsd.org> References: <50587F8D.9060102@FreeBSD.org> <505AD2A5.6060008@freebsd.org> <CAJ-FndBGEgkrKJ9bNdq0QrdyYZb=LXUsAG3wz5Lp-HLUBd5d9w@mail.gmail.com> <5099C5C9.4040703@freebsd.org>
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On Wed, 7 Nov 2012, David Xu wrote:
> 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.
>From ~2007
http://people.freebsd.org/~jeff/wakeupflags.diff
This has some different optimizations that can be done when you have a
hinted wakeup.
Thanks,
Jeff
>
>> Attilio
>>
>>
>
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