Date: Tue, 6 Nov 2012 11:03:47 +0000 From: Attilio Rao <attilio@freebsd.org> To: David Xu <davidxu@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: <CAJ-FndBGEgkrKJ9bNdq0QrdyYZb=LXUsAG3wz5Lp-HLUBd5d9w@mail.gmail.com> In-Reply-To: <505AD2A5.6060008@freebsd.org> References: <50587F8D.9060102@FreeBSD.org> <505AD2A5.6060008@freebsd.org>
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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.
Attilio
--
Peace can only be achieved by understanding - A. Einstein
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