On Mon, May 27, 2013 at 4:47 PM, Daniel Vetter daniel@ffwll.ch wrote:
On Mon, May 27, 2013 at 10:21 AM, Peter Zijlstra peterz@infradead.org wrote:
On Wed, May 22, 2013 at 07:24:38PM +0200, Maarten Lankhorst wrote:
+static inline void ww_acquire_init(struct ww_acquire_ctx *ctx,
struct ww_class *ww_class)+{
- ctx->task = current;
- do {
ctx->stamp = atomic_long_inc_return(&ww_class->stamp);- } while (unlikely(!ctx->stamp));
I suppose we'll figure something out when this becomes a bottleneck. Ideally we'd do something like:
ctx->stamp = local_clock();
but for now we cannot guarantee that's not jiffies, and I suppose that's a tad too coarse to work for this.
This might mess up when 2 cores happen to return exactly the same time, how do you choose a winner in that case? EDIT: Using pointer address like you suggested below is fine with me. ctx pointer would be static enough.
Right, but for now I suppose the 'global' atomic is ok, if/when we find it hurts performance we can revisit. I was just spewing ideas :-)
We could do a simple
ctx->stamp = (local_clock() << nr_cpu_shift) | local_processor_id()
to work around any bad luck in grabbing the ticket. With sufficient fine clocks the bias towards smaller cpu ids would be rather irrelevant. Just wanted to drop this idea before I'll forget about it again ;-)
Not a good idea to throw around random ideas right after a work-out. This is broken since different threads could end up with the same low bits. Comparing ctx pointers otoh on top of the timestamp should work. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch