linux-kernel - Re: tree rcu: call_rcu scalability problem?

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Message-Id: <1251919064.10394.25.camel@laptop>
Date:	Wed, 02 Sep 2009 21:17:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Paul McKenney <paulmck@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: tree rcu: call_rcu scalability problem?

On Wed, 2009-09-02 at 14:27 +0200, Nick Piggin wrote:

> It seems like nearly 2/3 of the cost is here:
>         /* Add the callback to our list. */
>         *rdp->nxttail[RCU_NEXT_TAIL] = head; <<<
>         rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
> 
> In loading the pointer to the next tail pointer. If I'm reading the profile
> correctly. Can't see why that should be a probem though...
> 
> ffffffff8107dee0 <__call_rcu>: /* __call_rcu total: 320971 100.000 */
>    697  0.2172 :ffffffff8107dee0:       push   %r12

>    921  0.2869 :ffffffff8107df57:       push   %rdx
>    151  0.0470 :ffffffff8107df58:       popfq
> 183507 57.1725 :ffffffff8107df59:       mov    0x50(%rbx),%rax
>    995  0.3100 :ffffffff8107df5d:       mov    %rdi,(%rax)

I'd guess at popfq to be the expensive op here.. skid usually causes the
attribution to be a few ops down the line.

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