Messages in this thread | | | Date | Wed, 7 Mar 2018 13:26:07 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT |
| |
On Wed, Mar 07, 2018 at 11:47:11AM +0000, Patrick Bellasi wrote: > On 06-Mar 20:02, Peter Zijlstra wrote: > > On Thu, Feb 22, 2018 at 05:01:50PM +0000, Patrick Bellasi wrote: > > > +struct util_est { > > > + unsigned int enqueued; > > > + unsigned int ewma; > > > +#define UTIL_EST_WEIGHT_SHIFT 2 > > > +}; > > > > > + ue = READ_ONCE(p->se.avg.util_est); > > > > > + WRITE_ONCE(p->se.avg.util_est, ue); > > > > That is actually quite dodgy... and relies on the fact that we have the > > 8 byte case in __write_once_size() and __read_once_size() > > unconditionally. It then further relies on the compiler DTRT for 32bit > > platforms, which is generating 2 32bit loads/stores. > > > > The advantage is of course that it will use single u64 loads/stores > > where available. > > Yes, that's mainly an "optimization" for 64bit targets... but perhaps > the benefits are negligible. > > Do you prefer to keep more "under control" the generated code by using > two {READ,WRITE}_ONCEs? > > IMO here we can also go with just the WRITE_ONCEs. I don't see a case > for the compiler to mangle load/store. While the WRITE_ONCE are still > required to sync with non rq-lock serialized code. > But... maybe I'm missing something... ?
I'm not sure we rely on READ/WRITE_ONCE() of 64bit variables on 32bit targets to be sane anywhere else (we could be, I just dont know).
I suspect it all works as expected... but its a tad tricky.
| |