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

From: Mike Galbraith
Date: Sat Jun 15 2013 - 01:49:12 EST

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.


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