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

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Date:	Thu, 27 Dec 2012 10:49:10 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, aquini@...hat.com,
	lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay
 factor

On Thu, 2012-12-27 at 09:35 -0500, Rik van Riel wrote:

> 
> The lock acquisition time depends on the holder of the lock,
> and what the CPUs ahead of us in line will do with the lock,
> not on the caller IP of the spinner.
> 

That would be true only for general cases.

In network land, we do have spinlock acquisition time depending on the
context.

A garbage collector usually runs for longer time than the regular fast
path.

But even without gc, its pretty often we have consumer/producers that
don't have the same amount of work to perform per lock/unlock sections.

The socket lock per example, might be held for very small sections for
process contexts (lock_sock() / release_sock()), but longer sections
from softirq context. Of course, severe lock contention on a socket
seems unlikely in real workloads.

> Therefore, I am not convinced that hashing on the caller IP
> will add much, if anything, except increasing the chance
> that we end up not backing off when we should...
> 
> IMHO it would be good to try keeping this solution as simple
> as we can get away with.
> 

unsigned long hash = (unsigned long)lock ^
                     (unsigned long)__builtin_return_address(1);

seems simple enough to me, but I get your point.

I also recorded the max 'delay' value reached on my machine to check how
good MAX_SPINLOCK_DELAY value was :

[   89.628265] cpu 16 delay 3710
[   89.631230] cpu 6 delay 2930
[   89.634120] cpu 15 delay 3186
[   89.637092] cpu 18 delay 3789
[   89.640071] cpu 22 delay 4012
[   89.643080] cpu 11 delay 3389
[   89.646057] cpu 21 delay 3123
[   89.649035] cpu 9 delay 3295
[   89.651931] cpu 3 delay 3063
[   89.654811] cpu 14 delay 3335

Although it makes no performance difference to use a bigger/smaller one.

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