linux-kernel - Re: AIM7 40% regression with 2.6.26-rc1

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Date:	Tue, 6 May 2008 19:39:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Matthew Wilcox <matthew@....cx>,
	"J. Bruce Fields" <bfields@...i.umich.edu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: AIM7 40% regression with 2.6.26-rc1


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> Finally: how come we regressed by swapping the semaphore 
> implementation anyway?  We went from one sleeping lock implementation 
> to another - I'd have expected performance to be pretty much the same.
> 
> <looks at the implementation>
> 
> down(), down_interruptible() and down_try() should use 
> spin_lock_irq(), not irqsave.
> 
> up() seems to be doing wake-one, FIFO which is nice.  Did the 
> implementation which we just removed also do that?  Was it perhaps 
> accidentally doing LIFO or something like that?

i just checked the old implementation on x86. It used 
lib/semaphore-sleepers.c which does one weird thing:

  - __down() when it returns wakes up yet another task via 
    wake_up_locked().

i.e. we'll always keep yet another task in flight. This can mask wakeup 
latencies especially when it takes time.

The patch (hack) below tries to emulate this weirdness - it 'kicks' 
another task as well and keeps it busy. Most of the time this just 
causes extra scheduling, but if AIM7 is _just_ saturating the number of 
CPUs, it might make a difference. Yanmin, does the patch below make any 
difference to the AIM7 results?

( it would be useful data to get a meaningful context switch trace from 
  the whole regressed workload, and compare it to a context switch trace 
  with the revert added. )

	Ingo

---
 kernel/semaphore.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

Index: linux/kernel/semaphore.c
===================================================================
--- linux.orig/kernel/semaphore.c
+++ linux/kernel/semaphore.c
@@ -261,4 +261,14 @@ static noinline void __sched __up(struct
 	list_del(&waiter->list);
 	waiter->up = 1;
 	wake_up_process(waiter->task);
+
+	if (likely(list_empty(&sem->wait_list)))
+		return;
+	/*
+	 * Opportunistically wake up another task as well but do not
+	 * remove it from the list:
+	 */
+	waiter = list_first_entry(&sem->wait_list,
+				  struct semaphore_waiter, list);
+	wake_up_process(waiter->task);
 }
--
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