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Message-ID: <yyimz065x1a.wl@toshiba.co.jp>
Date: Tue, 15 May 2007 17:08:33 +0900
From: Tsutomu OWA <tsutomu.owa@...hiba.co.jp>
To: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Arnd Bergmann <arnd@...db.de>, linuxppc-dev@...abs.org,
Thomas Gleixner <tglx@...utronix.de>, mingo@...e.hu,
linux-kernel@...r.kernel.org
Subject: Re: [patch 4/4] powerpc 2.6.21-rt1: reduce scheduling latency by changing tlb flush size
At Tue, 15 May 2007 17:38:27 +1000, Benjamin Herrenschmidt wrote:
> > +#ifdef CONFIG_PREEMPT_RT
> > + /*
> > + * Since flushing tlb needs expensive hypervisor call(s) on celleb,
> > + * always flush it on RT to reduce scheduling latency.
> > + */
> > + if (machine_is(celleb)) {
> > + flush_tlb_pending();
> > + return;
> > + }
> > +#endif /* CONFIG_PREEMPT_RT */
> Any reason to do that only on celleb ? :-)
Well, at least it does not harm any other platforms :)
> Also, we might want to still batch, though in smaller amount.
Yeah, it's a good point.
> Have you measured
> the time it takes ? We might want to modulate the amount based on wether we
> are using native hash tables or an hypervisor.
Yes, here is the trace log. Accordint it, flushing 9 entries takes about 50us.
It means that flushing 192 (PPC64_TLB_BATCH_NR) entries takes 1ms.
Please note that tracing causes *roughly* 20-30% overhead, though.
I'm afraid I don't have numbers w/ native version, but I suppose you have :)
# cat /proc/latency_trace
preemption latency trace v1.1.5 on 2.6.21-rt1
--------------------------------------------------------------------
latency: 60 us, #53/53, CPU#0 | (M:rt VP:0, KP:0, SP:1 HP:1 #P:1)
-----------------
| task: inetd-358 (uid:0 nice:0 policy:0 rt_prio:0)
-----------------
=> started at: .__switch_to+0xa8/0x178 <c00000000000feec>
=> ended at: .__start+0x4000000000000000/0x8 <00000000>
_------=> CPU#
/ _-----=> irqs-off
| / _----=> need-resched
|| / _---=> hardirq/softirq
||| / _--=> preempt-depth
|||| /
||||| delay
cmd pid ||||| time | caller
\ / ||||| \ | /
inetd-358 0D..2 0us : .user_trace_start (.__switch_to)
inetd-358 0D..2 0us : .rt_up (.user_trace_start)
inetd-358 0D..3 1us : .rt_mutex_unlock (.rt_up)
inetd-358 0D..3 2us : .__flush_tlb_pending (.__switch_to)
inetd-358 0D..4 3us : .flush_hash_range (.__flush_tlb_pending)
inetd-358 0D..4 3us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 4us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 5us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 6us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 13us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 13us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 14us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 14us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 15us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 18us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 19us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 20us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 20us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 21us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 24us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 25us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 25us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 26us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 26us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 30us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 30us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 31us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 31us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 32us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 36us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 36us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 37us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 37us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 38us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 41us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 42us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 42us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 43us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 43us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 47us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 48us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 48us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 49us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 49us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 52us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..4 53us : .flush_hash_page (.flush_hash_range)
inetd-358 0D..4 54us : .beat_lpar_hpte_invalidate (.flush_hash_page)
inetd-358 0D..4 54us : .__spin_lock_irqsave (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 55us+: .beat_lpar_hpte_getword0 (.beat_lpar_hpte_invalidate)
inetd-358 0D..5 58us : .__spin_unlock_irqrestore (.beat_lpar_hpte_invalidate)
inetd-358 0D..2 59us : .user_trace_stop (.__switch_to)
inetd-358 0D..2 60us : .user_trace_stop (.__switch_to)
-- owa
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