Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups

On Wed, Sep 12, 2018 at 06:52:02PM +0100, Patrick Bellasi wrote:

On 12-Sep 19:42, Peter Zijlstra wrote:

quoted

On Wed, Sep 12, 2018 at 06:35:15PM +0100, Patrick Bellasi wrote:

quoted

On 12-Sep 18:12, Peter Zijlstra wrote:

quoted

quoted

No idea; but if you want to go all fancy you can replace he whole
uclamp_map thing with something like:

struct uclamp_map {
	union {
		struct {
			unsigned long v : 10;
			unsigned long c : BITS_PER_LONG - 10;
		};
		atomic_long_t s;
	};
};

That sounds really cool and scary at the same time :)

The v:10 requires that we never set SCHED_CAPACITY_SCALE>1024
or that we use it to track a percentage value (i.e. [0..100]).

Or we pick 11 bits, it seems unlikely that capacity be larger than 2k.

Just remembered a past experience where we had unaligned access traps
on some machine because... don't you see any potentially issue on
using bitfleds like you suggest above ?

I'm thinking to:

   commit 317d359df95d ("sched/core: Force proper alignment of 'struct util_est'")

There should not be (and I'm still confused by that particular commit
you reference). If we access everything through the uclamp_map::s, and
only use the bitfields to interpret the results, it all 'works'.

The tricky thing we did earlier was trying to use u64 accesses for 2
u32 variables. And somehow ia64 didn't get the alignment right.