On Mon, Oct 17, 2016 at 11:14:11AM +0200, Vincent Guittot wrote:
/*
- Signed add and clamp on underflow.
- Explicitly do a load-store to ensure the intermediate value never hits
- memory. This allows lockless observations without ever seeing the negative
- values.
- */
+#define add_positive(_ptr, _val) do { \
- typeof(_ptr) ptr = (_ptr); \
- typeof(_val) res, val = (_val); \
- typeof(*ptr) var = READ_ONCE(*ptr); \
- res = var + val; \
- if (res < 0) \
res = 0; \
I think this is broken, and inconsistent with sub_positive().
The thing is, util_avg, on which you use this, is an unsigned type. Checking for unsigned underflow can be done by comparing against either one of the terms.
- WRITE_ONCE(*ptr, res); \
+} while (0)
- add_positive(&cfs_rq->avg.util_avg, delta);