On Tue, Feb 18, 2014 at 07:50:13PM -0500, Waiman Long wrote:
On 02/18/2014 04:34 PM, Peter Zijlstra wrote:Have you actually measured this?
On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:The main reason for using xchg instead of cmpxchg is its performance impact
The #ifdef is harder to take away here. The point is that doing a 32-bitDoes it really pay to use xchg() with all those fixup cases? Why not
exchange may accidentally steal the lock with the additional code to handle
that. Doing a 16-bit exchange, on the other hand, will never steal the lock
and so don't need the extra handling code. I could construct a function with
different return values to handle the different cases if you think it will
make the code easier to read.
have a single cmpxchg() loop that does just the exact atomic op you
want?
when the lock is heavily contended. Under those circumstances, a task may
need to do several tries of read+atomic-RMV before getting it right. This
may cause a lot of cacheline contention. With xchg, we need at most 2 atomic
ops. Using cmpxchg() does simplify the code a bit at the expense of
performance with heavy contention.