Messages in this thread | | | Date | Fri, 15 Dec 2017 15:07:51 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 2/4] sched/fair: add util_est on top of PELT |
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On Fri, Dec 15, 2017 at 02:02:18PM +0000, Patrick Bellasi wrote: > On 13-Dec 17:05, Peter Zijlstra wrote: > > On Tue, Dec 05, 2017 at 05:10:16PM +0000, Patrick Bellasi wrote: > > > + if (cfs_rq->nr_running > 0) { > > > + util_est = cfs_rq->util_est_runnable; > > > + util_est -= task_util_est(p); > > > + if (util_est < 0) > > > + util_est = 0; > > > + cfs_rq->util_est_runnable = util_est; > > > + } else { > > > > I'm thinking that's an explicit load-store to avoid intermediate values > > landing in cfs_rq->util_esp_runnable, right? > > Was mainly to have an unsigned util_est for the following "sub"... > > > > That would need READ_ONCE() / WRITE_ONCE() I think, without that the > > compiler is free to munge the lot together. > > ... do we still need the {READ,WRITE}_ONCE() in this case? > I guess adding them however does not hurts.
I think the compiler is free to munge it into something like:
cfs_rq->util_est_runnable -= task_util_est(); if (cfs_rq->util_est_runnable < 0) cfs_rq->util_est_runnable = 0
and its a fairly simple optimization at that, it just needs to observe that util_est is an alias for cfs_rq->util_est_runnable.
Using the volatile load/store completely destroys that optimization though, so yes, I'd say its definitely needed.
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