Messages in this thread | | | Date | Thu, 27 Jul 2006 01:53:55 -0400 | From | Chuck Ebbert <> | Subject | Re: + spinlock_debug-dont-recompute-jiffies_per_loop.patch added to -mm tree |
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In-Reply-To: <20060725204306.GA22547@elte.hu>
On Tue, 25 Jul 2006 22:43:06 +0200, Ingo Molnar wrote: > > * Dave Jones <davej@redhat.com> wrote: > > > > iteration limit, gets recomputed every time. Caching it explicitly > > > prevents that. > > > > What is the purpose of those __delays being there at all ? Seems odd > > to be waiting that long when the spinlock could become available a lot > > sooner. (These also make spinlock debug really painful on boxes with > > huge numbers of CPUs). > > the debug code has to figure out when to trigger a deadlock warning > message. If we are looping in a deadlock with irqs disabled on all CPUs, > there's nothing that advances jiffies. The TSC is not reliable. The > thing that remains is to use __delay(1). We could calibrate the loop > separately perhaps?
Is there some reason this code:
for (i = 0; i < loops_per_jiffy * HZ; i++) { if (__raw_spin_trylock(&lock->raw_lock)) return; __delay(1); }
needs to continuously try to update the spinlock? Shouldn't it just read it first, like this, to avoid the bus update traffic?
if (spin_can_lock(&lock->raw_lock) && __raw_spin_trylock(&lock->raw_lock)) return;
Also, looking at __delay(), I foresee problems on i386 with the HPET timer. Every call to __delay() causes at least two HPET timer reads and it looks like they're slow (using readl() on ioremapped memory, anyway.)
-- Chuck
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