linux-kernel - Re: [rfc][patch 2/2] x86

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Date:	Mon, 16 Jul 2007 12:18:28 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <ak@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc][patch 2/2] x86_64: FIFO ticket spinlocks

On Mon, Jul 16, 2007 at 11:49:40AM +0200, Ingo Molnar wrote:
> 
> * Nick Piggin <npiggin@...e.de> wrote:
> 
> > On Mon, Jul 16, 2007 at 11:26:46AM +0200, Ingo Molnar wrote:
> > > 
> > > * Nick Piggin <npiggin@...e.de> wrote:
> > > 
> > > > [...] trylock is more significantly slower, but they are relatively 
> > > > rare.
> > > 
> > > trylock is the main thing that the spinlock debugging code uses, and 
> > > SPINLOCK_DEBUG is frequently enabled by distro kernels. OTOH, the cost 
> > > looks like to be +5 instructions, right? Still ...
> > 
> > Which trylocks do you mean? The lockbreak spinlocks use trylock, but 
> > those are not used with the ticket version.
> 
> the trylocks in lib/spinlock-debug.c:
> 
>  static void __spin_lock_debug(spinlock_t *lock)
>  {
>  ...
>                          if (__raw_spin_trylock(&lock->raw_lock))
>                                  return;
>  ...
>  void _raw_spin_lock(spinlock_t *lock)
>  {
>          debug_spin_lock_before(lock);
>          if (unlikely(!__raw_spin_trylock(&lock->raw_lock)))
>                  __spin_lock_debug(lock);
>          debug_spin_lock_after(lock);
>  }
> 
> am i missing something?

No, I missed that. Yeah, that would get a bit slower, but I'm not sure
if it would be a problem on a kernel where you have spinlock debuggin
on anyway.

If it becomes a problem, we could perhaps do a version for ticket locks
that first takes a ticket, and then is for up to a second before
printing the stuck lock message. That would make the performance hit go away.

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