linux-kernel - Re: [RFC] time: xtime

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Date:	Thu, 5 May 2011 23:01:18 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	john stultz <johnstul@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Anton Blanchard <anton@...ba.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC] time: xtime_lock is held too long

> > Another idea would be to prime cache lines to be dirtied in cpu cache
> > before taking locks, and better pack variables to reduce number of cache
> > lines.
> 
> Most variables are packed already in struct timekeeper, which should
> be pretty cache hot anyway, so I don't know whether we gain much.

There's actually some potential here. I got a moderate speedup in a
database benchmark with this patch recently. The biggest win
was in the timer interrupt.

All those variables are on separate cache lines.

(needs some cleanups, just for illustration)

-Andi

From: Andi Kleen <ak@...ux.intel.com>
Subject: [PATCH] do prefetches for timer state in timer interrupt

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 76b96d7..2704267 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -856,6 +856,17 @@ void __irq_entry smp_apic_timer_interrupt(struct pt_regs *regs)
 {
 	struct pt_regs *old_regs = set_irq_regs(regs);
 
+	extern struct timespec xtime;
+	extern struct timespec wall_to_monotonic;
+
+
+	/* Prefetch here to make the ktime_get later faster.
+ 	 * XXX move them all on the same cache line.
+ 	 */
+	prefetch(&xtime_lock);
+	prefetch(&xtime);
+	prefetch(&wall_to_monotonic);
+
 	/*
 	 * NOTE! We'd better ACK the irq immediately,
 	 * because timer handling can be slow.
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d27c756..0002e9e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -157,8 +157,8 @@ __cacheline_aligned_in_smp DEFINE_SEQLOCK(xtime_lock);
  * - wall_to_monotonic is no longer the boot time, getboottime must be
  * used instead.
  */
-static struct timespec xtime __attribute__ ((aligned (16)));
-static struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
+struct timespec xtime __attribute__ ((aligned (16)));
+struct timespec wall_to_monotonic __attribute__ ((aligned (16)));
 static struct timespec total_sleep_time;
 
 /*

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