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List:       linux-kernel
Subject:    Re: long delays (possibly infinite) in time_interpolator_get_counter
From:       Christoph Lameter <clameter () engr ! sgi ! com>
Date:       2005-07-31 20:00:12
Message-ID: Pine.LNX.4.62.0507311256440.32674 () schroedinger ! engr ! sgi ! com
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On Sun, 31 Jul 2005, Alex Williamson wrote:

>    Ok, here's an optimization that should help reduce contention on the
> cmpxchg, has zero impact on the nojitter path, and doesn't require any
> changes to fsys.  When a caller already had the xtime_lock write lock
> there's no need to fight with other CPUs for the cmpxchg.  The other

Yup correct.

> "reader" CPUs will have to fetch it again since a seqlock write is in
> progress.  Therefore we can simplify this path as shown below.  The
> write is atomic, and we don't care if another CPU has changed last_cycle
> since it can't return the value until the write lock is released.  This
> has only been compile tested, but I'm interested to hear your opinion.

time_interpolator_get_counter is static inline. So you may modify the
existing function and pass a constant parameter without a 
performance reduction. Two different versions will then be generated for 
time_interpolator_get_counter at compile time.

Maybe call the parameter "writelock"?

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