Messages in this thread | | | Date | Mon, 9 Nov 2020 13:41:39 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH v5 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET flag to reset uclamp |
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On 11/08/20 03:15, Yun Hsiang wrote: > I think SCHED_FLAG_ALL is for internal kernel use. So I agree with not > exporting it to user.
+1 for the #ifdef __kernel__
> > > + if (!(flags & SCHED_FLAG_UTIL_CLAMP_RESET)) > > > + return false; > > > + > > > + /* Only _UCLAMP_RESET flag set: reset both clamps */ > > > + if (!(flags & (SCHED_FLAG_UTIL_CLAMP_MIN | SCHED_FLAG_UTIL_CLAMP_MAX))) > > > + return true; > > > + > > > + /* Both _UCLAMP_RESET and _UCLAMP_MIN flags are set: reset only min */ > > > + if ((flags & SCHED_FLAG_UTIL_CLAMP_MIN) && clamp_id == UCLAMP_MIN) > > > + return true; > > > + > > > + /* Both _UCLAMP_RESET and _UCLAMP_MAX flags are set: reset only max */ > > > + if ((flags & SCHED_FLAG_UTIL_CLAMP_MAX) && clamp_id == UCLAMP_MAX) > > > + return true; > > > + > > > + return false; > > > > I was suggesting maybe we need READ_ONCE() in the if above but did not > > addressed previous Dietmar's question [2] on why. > > > > The function above has a correct semantics only when the ordering of the > > if statement is strictly observed. > > > > Now, since we don't have any data dependency on the multiple if cases, > > are we sure an overzealous compiler will never come up with some > > "optimized reordering" of the checks required? > > > > IOW, if the compiler could scramble the checks on an optimization, we > > would get a wrong semantics which is also very difficult do debug. > > Hence the idea to use READ_ONCE, to both tell the compiler to not > > even attempt reordering those checks and also to better document the > > code about the importance of the ordering on those checks. > > > > Is now more clear? Does that makes sense? > > I can undertand what your worries. But I'm not sure is it really needed. > If there is no other concern I can add it.
I too don't think the compiler has a power to do such an optimization. It must preserve the order of the checks even if it found a more efficient way to perform them.
Thanks
-- Qais Yousef
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