linux-kernel - [PATCH 0/3] ipc/sem.c: Optimization for reducing spinlock contention

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Message-Id: <1272481588-1941-1-git-send-email-manfred@colorfullife.com>
Date:	Wed, 28 Apr 2010 21:06:25 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Zach Brown <zach.brown@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 0/3] ipc/sem.c: Optimization for reducing spinlock contention

Hi,

I've cleaned up the patches I sent a week ago:
- they pass now checkpatch.pl
- some comments updated/added
- a bug with semctl(,,SETALL,) is fixed
- tested against LTP

Andrew: Could you add them the next -mm?

The following series of patches tries to fix the spinlock contention
reported by Chris Manson: His benchmark exposes problems of the current
code:

- In the worst case, the algorithm used by update_queue() is O(N^2).
  Bulk wake-up calls can enter this worst case.
  The patch series fix that.
  Note that the benchmark app doesn't expose the problem, it just should
  be fixed: Real world apps might do the wake-ups in another order
  than perfect FIFO.

- The part of the code that runs within the semaphore array spinlock
  is significantly larger than necessary.
  The patch series fixes that. This change is responsible for the
  main improvement.

- The cacheline with the spinlock is also used for a variable that
  is read in the hot path (sem_base) and for a variable that is
  unnecessarily written to multiple times (sem_otime).
  The last step of the series cacheline-aligns the spinlock.

--
	Manfred
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