Roman Zippel wrote:
Hi,
On Thu, 15 Sep 2005, Nick Piggin wrote:
Roman: any ideas about what you would prefer? You'll notice
atomic_inc_not_zero replaces rcuref_inc_lf, which is used several times
in the VFS.
In the larger picture I'm not completely happy with these scalibilty
patches, as they add extra overhead at the lower end. On a UP system in
general nothing beats:
spin_lock();
if (*ptr)
ptr += 1;
spin_unlock();
The main problem is here that the atomic functions are used in two basic
situation:
1) interrupt synchronization
2) multiprocessor synchronization
The atomic functions have to assume both, but on UP systems it often is
a lot cheaper if they don't have to synchronize with interrupts. So
replacing a spinlock with a few atomic operations can hurt UP performance.
Maybe so, but what I'm doing is introducing a slightly better
implementation of what is currently in tree, and attempting to
follow current standards as far as possible. I don't think you
could say that is a bad thing.
Now I don't think anyone would be flat out opposed to 1 - reworking
the atomic.h code to allow some genericity (is that a word?); 2 -
reworking atomic.h code to allow combining of atomic ops, or allowing
interrupt unsafe ops...
Of course, neither is going to be merged unless done tastefully, and
I imagine both would be difficult to get right, with probably a low
cost/benefit ratio.
Thanks,
Nick
--
SUSE Labs, Novell Inc.
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