Date: Wed, 19 Sep 2012 09:03:05 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: attilio@FreeBSD.org Cc: freebsd-hackers <freebsd-hackers@FreeBSD.org>, Jeff Roberson <jeff@FreeBSD.org> Subject: Re: ule+smp: small optimization for turnstile priority lending Message-ID: <50596019.5060708@FreeBSD.org> In-Reply-To: <CAJ-FndC8j12a95N0QprYTLJLC06R8jjcaHuxEZKu5D9RHW=ZVw@mail.gmail.com> References: <50587F8D.9060102@FreeBSD.org> <CAJ-FndCWzTBRYsA0mFDCj8RU06ZUTi3G0LeEFcS9_c5zKd%2BgWQ@mail.gmail.com> <5058C68B.1010508@FreeBSD.org> <CAJ-FndC8j12a95N0QprYTLJLC06R8jjcaHuxEZKu5D9RHW=ZVw@mail.gmail.com>
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on 19/09/2012 02:01 Attilio Rao said the following:
> On Tue, Sep 18, 2012 at 8:07 PM, Andriy Gapon <avg@freebsd.org> wrote:
>> on 18/09/2012 19:50 Attilio Rao said the following:
>>> On 9/18/12, Andriy Gapon <avg@freebsd.org> 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.
>>>
>>> I think that the idea is bright, but I have reservations against the
>>> implementation because it seems to me there are too many layering
>>> violations.
>>
>> Just one - for a layer between tunrstile and scheduler :-)
>> But I agree.
>>
>>> What is suggest is somewhat summarized like that:
>>> - Add a new SRQ_WILLSLEEP or the name you prefer
>>> - Add a new "flags" argument to sched_lend_prio() (both ule and 4bsd)
>>> and sched_thread_priority (ule only)
>>> - sched_thread_priority() will pass down the new flag to sched_add()
>>> which passed down to sched_pickcpu().
>>>
>>> This way sched_pickcpu() has the correct knowledge of what is going on
>>> and it can make the right decision. You likely don't need to lower the
>>> tdq_load at that time either this way, because sched_pickcpu() can
>>> just adjust it locally for its decision.
>>>
>>> What do you think?
>>
>> This sounds easy but it is not quite so given the implementation of
>> sched_pickcpu and sched_lowest. This is probably more work than I am able to
>> take now.
>
> I think actually that the attached patch should do what you need. Of
> course there is more runqueue locking, due to the tdq_load fondling,
> but I'm not sure it is really a big deal.
> I didn't test it yet, as I understand you have a test case, so maybe
> you can. However if Jeff agrees I can send the patch to flo@ for
> performance evaluation.
>
> Thoughts?
Originally I had a similar patch with a small difference that I did tdq_load
manipulations in sched_thread_priority around sched_add call. That patch
produced a deadlock because of the need for TDQ_LOCK.
> Index: sys/sys/sched.h
> ===================================================================
> --- sys/sys/sched.h (revisione 240672)
> +++ sys/sys/sched.h (copia locale)
> @@ -91,7 +91,7 @@ void sched_nice(struct proc *p, int nice);
> */
> void sched_exit_thread(struct thread *td, struct thread *child);
> void sched_fork_thread(struct thread *td, struct thread *child);
> -void sched_lend_prio(struct thread *td, u_char prio);
> +void sched_lend_prio(struct thread *td, u_char prio, int flags);
> void sched_lend_user_prio(struct thread *td, u_char pri);
> fixpt_t sched_pctcpu(struct thread *td);
> void sched_prio(struct thread *td, u_char prio);
> @@ -161,6 +161,7 @@ sched_unpin(void)
> #define SRQ_INTR 0x0004 /* It is probably urgent. */
> #define SRQ_PREEMPTED 0x0008 /* has been
> preempted.. be kind */
> #define SRQ_BORROWING 0x0010 /* Priority updated
> due to prio_lend */
> +#define SRQ_WILLSWITCH 0x0020 /* curthread will be
> switched out */
>
> /* Scheduler stats. */
> #ifdef SCHED_STATS
> Index: sys/kern/sched_ule.c
> ===================================================================
> --- sys/kern/sched_ule.c (revisione 240672)
> +++ sys/kern/sched_ule.c (copia locale)
> @@ -290,7 +290,7 @@ static struct tdq tdq_cpu;
> #define TDQ_LOCKPTR(t) (&(t)->tdq_lock)
>
> static void sched_priority(struct thread *);
> -static void sched_thread_priority(struct thread *, u_char);
> +static void sched_thread_priority(struct thread *, u_char, int flags);
> static int sched_interact_score(struct thread *);
> static void sched_interact_update(struct thread *);
> static void sched_interact_fork(struct thread *);
> @@ -1233,6 +1233,18 @@ sched_pickcpu(struct thread *td, int flags)
> }
> }
> /*
> + * If SRQ_WILLSWITCH is set, this means curthread on the currcpu
> + * is going to be switched out very soon. This means we should
> + * consider the curcpu as less loaded than expected.
> + * tdq_load is fondled directly, rather than calling tdq_load_rem(),
> + * because there is no need to handle tdq_sysload for this purpose.
> + */
> + if (flags & SRQ_WILLSWITCH) {
> + TDQ_LOCK(TDQ_CPU(self));
> + TDQ_CPU(self)->tdq_load--;
> + TDQ_UNLOCK(TDQ_CPU(self));
> + }
> + /*
> * Search for the last level cache CPU group in the tree.
> * Skip caches with expired affinity time and SMT groups.
> * Affinity to higher level caches will be handled less aggressively.
> @@ -1270,6 +1282,11 @@ sched_pickcpu(struct thread *td, int flags)
> cpu = self;
> } else
> SCHED_STAT_INC(pickcpu_lowest);
> + if (flags & SRQ_WILLSWITCH) {
> + TDQ_LOCK(TDQ_CPU(self));
> + TDQ_CPU(self)->tdq_load++;
> + TDQ_UNLOCK(TDQ_CPU(self));
> + }
> if (cpu != ts->ts_cpu)
> SCHED_STAT_INC(pickcpu_migration);
> return (cpu);
> @@ -1629,7 +1646,7 @@ sched_pctcpu_update(struct td_sched *ts, int run)
> * functions.
> */
> static void
> -sched_thread_priority(struct thread *td, u_char prio)
> +sched_thread_priority(struct thread *td, u_char prio, int flags)
> {
> struct td_sched *ts;
> struct tdq *tdq;
> @@ -1659,7 +1676,7 @@ static void
> if (TD_ON_RUNQ(td) && prio < td->td_priority) {
> sched_rem(td);
> td->td_priority = prio;
> - sched_add(td, SRQ_BORROWING);
> + sched_add(td, flags | SRQ_BORROWING);
> return;
> }
> /*
> @@ -1684,11 +1701,11 @@ static void
> * priority.
> */
> void
> -sched_lend_prio(struct thread *td, u_char prio)
> +sched_lend_prio(struct thread *td, u_char prio, int flags)
> {
>
> td->td_flags |= TDF_BORROWING;
> - sched_thread_priority(td, prio);
> + sched_thread_priority(td, prio, flags);
> }
>
> /*
> @@ -1711,9 +1728,9 @@ sched_unlend_prio(struct thread *td, u_char prio)
> base_pri = td->td_base_pri;
> if (prio >= base_pri) {
> td->td_flags &= ~TDF_BORROWING;
> - sched_thread_priority(td, base_pri);
> + sched_thread_priority(td, base_pri, 0);
> } else
> - sched_lend_prio(td, prio);
> + sched_lend_prio(td, prio, 0);
> }
>
> /*
> @@ -1736,7 +1753,7 @@ sched_prio(struct thread *td, u_char prio)
>
> /* Change the real priority. */
> oldprio = td->td_priority;
> - sched_thread_priority(td, prio);
> + sched_thread_priority(td, prio, 0);
>
> /*
> * If the thread is on a turnstile, then let the turnstile update
> Index: sys/kern/sched_4bsd.c
> ===================================================================
> --- sys/kern/sched_4bsd.c (revisione 240672)
> +++ sys/kern/sched_4bsd.c (copia locale)
> @@ -859,7 +859,7 @@ sched_priority(struct thread *td, u_char prio)
> * priority.
> */
> void
> -sched_lend_prio(struct thread *td, u_char prio)
> +sched_lend_prio(struct thread *td, u_char prio, int flags __unused)
> {
>
> td->td_flags |= TDF_BORROWING;
> @@ -888,7 +888,7 @@ sched_unlend_prio(struct thread *td, u_char prio)
> td->td_flags &= ~TDF_BORROWING;
> sched_prio(td, base_pri);
> } else
> - sched_lend_prio(td, prio);
> + sched_lend_prio(td, prio, 0);
> }
>
> void
> Index: sys/kern/subr_turnstile.c
> ===================================================================
> --- sys/kern/subr_turnstile.c (revisione 240672)
> +++ sys/kern/subr_turnstile.c (copia locale)
> @@ -158,7 +158,7 @@ static void init_turnstile0(void *dummy);
> #ifdef TURNSTILE_PROFILING
> static void init_turnstile_profiling(void *arg);
> #endif
> -static void propagate_priority(struct thread *td);
> +static void propagate_priority(struct thread *td, int flags);
> static int turnstile_adjust_thread(struct turnstile *ts,
> struct thread *td);
> static struct thread *turnstile_first_waiter(struct turnstile *ts);
> @@ -178,9 +178,10 @@ SDT_PROBE_DEFINE2(sched, , , wakeup, wakeup, "stru
> * Walks the chain of turnstiles and their owners to propagate the priority
> * of the thread being blocked to all the threads holding locks that have to
> * release their locks before this thread can run again.
> + * It accepts SRQ_* informations as flags.
> */
> static void
> -propagate_priority(struct thread *td)
> +propagate_priority(struct thread *td, int flags)
> {
> struct turnstile *ts;
> int pri;
> @@ -240,7 +241,7 @@ static void
> /*
> * Bump this thread's priority.
> */
> - sched_lend_prio(td, pri);
> + sched_lend_prio(td, pri, flags);
>
> /*
> * If lock holder is actually running or on the run queue
> @@ -445,7 +446,7 @@ turnstile_adjust(struct thread *td, u_char oldpri)
> td->td_tsqueue == TS_SHARED_QUEUE);
> if (td == TAILQ_FIRST(&ts->ts_blocked[td->td_tsqueue]) &&
> td->td_priority < oldpri) {
> - propagate_priority(td);
> + propagate_priority(td, 0);
> }
> }
>
> @@ -659,7 +660,7 @@ turnstile_claim(struct turnstile *ts)
> */
> thread_lock(owner);
> if (td->td_priority < owner->td_priority)
> - sched_lend_prio(owner, td->td_priority);
> + sched_lend_prio(owner, td->td_priority, 0);
> thread_unlock(owner);
> tc = TC_LOOKUP(ts->ts_lockobj);
> mtx_unlock_spin(&ts->ts_lock);
> @@ -741,7 +742,7 @@ turnstile_wait(struct turnstile *ts, struct thread
> td->td_blktick = ticks;
> TD_SET_LOCK(td);
> mtx_unlock_spin(&tc->tc_lock);
> - propagate_priority(td);
> + propagate_priority(td, SRQ_WILLSWITCH);
>
> if (LOCK_LOG_TEST(lock, 0))
> CTR4(KTR_LOCK, "%s: td %d blocked on [%p] %s", __func__,
>
--
Andriy Gapon
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