linux-kernel - Re: [PATCH 0/6] ipc/sem.c: performance improvements, FIFO

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Date:	Sat, 15 Jun 2013 07:48:39 +0200
From:	Mike Galbraith <efault@....de>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>, hhuang@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/6] ipc/sem.c: performance improvements, FIFO

On Sat, 2013-06-15 at 07:27 +0200, Manfred Spraul wrote:

> Assume there is one op (semctl(), whatever) that acquires the global 
> lock - and a continuous stream of simple ops.
> - spin_is_locked() returns true due to the semctl().
> - then simple ops will switch to spin_lock(&sma->sem_perm.lock).
> - since the spinlock is acquired, the next operation will get true from 
> spin_is_locked().
> 
> It will stay that way around - as long as there is at least one op 
> waiting for sma->sem_perm.lock.
> With enough cpus, it will stay like this forever.

Yup, pondered that yesterday, scratching my head over how to do better.
Hints highly welcome.  Maybe if I figure out how to scratch dual lock
thingy properly for -rt, non-rt will start acting sane too, as that spot
seems to be itchy in both kernels.

-Mike

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