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.
/*
* Signal a taskqueue thread to terminate.
*/
static void
taskqueue_terminate(struct thread **pp, struct taskqueue *tq)
{
while (tq->tq_tcount > 0 || tq->tq_callouts > 0) {
wakeup(tq);
TQ_SLEEP(tq, pp, &tq->tq_mutex, PWAIT,
"taskqueue_destroy", 0);
}
}
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