Messages in this thread Patch in this message | | | Date | Thu, 5 May 2011 23:01:18 +0200 | From | Andi Kleen <> | Subject | Re: [RFC] time: xtime_lock is held too long |
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> > 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@linux.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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