Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2013-01-03 at 11:45 -0500, Steven Rostedt wrote:
> On Thu, 2013-01-03 at 08:10 -0800, Eric Dumazet wrote:
> 
> > > But then would the problem even exist? If the lock is on its own cache
> > > line, it shouldn't cause a performance issue if other CPUs are spinning
> > > on it. Would it?
> > 
> > Not sure I understand the question.
> > 
> 
> I'll explain my question better.
> 
> I thought the whole point of Rik's patches was to solve a performance
> problem caused by contention on a lock that shares a cache line with
> data.
> 
> In the ideal case, locks wont be contented, and are taken and released
> quickly (being from the RT world, I know this isn't true :-( ). In this
> case, it's also advantageous to keep the lock on the same cache line as
> the data that's being updated. This way, the process of grabbing the
> lock also pulls in the data that you will soon be using.
> 
> But then the problem occurs when you have a bunch of other CPUs trying
> to take this lock in a tight spin. Every time the owner of the lock
> touches the data, the other CPUs doing a LOCK read on the spinlock will
> cause bus contention on the owner CPU as the data shares the cache and
> needs to be synced. As the owner CPU just touched the cache line that is
> under a tight loop of LOCK reads on other CPUs. By adding the delays,
> the CPU with the lock doesn't stall at every update of the data
> protected by the lock.
> 
> Thus, if monitor/mwait is ideal only for locks on its own cache line,
> then they are pointless for the locks that are causing the issue we are
> trying to fix.

I think you misunderstood the monitor/mwait usage I was speaking of

- Only for MCS type lock, where each cpu spins on it own busy/locked
bit.

Of course, if we use a ticket spinlock with no additional storage, we
have to spin without making any memory reference, and thats Rick's patch
using this idea : 

http://www.cs.rochester.edu/research/synchronization/pseudocode/ss.html#ticket