Messages in this thread | | | Date | Thu, 9 Feb 2017 07:45:01 +0100 | From | Ingo Molnar <> | Subject | Re: tip: demise of tsk_cpus_allowed() and tsk_nr_cpus_allowed() |
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* Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Feb 08, 2017 at 11:20:19AM +0100, Thomas Gleixner wrote: > > On Mon, 6 Feb 2017, Ingo Molnar wrote: > > > * Peter Zijlstra <peterz@infradead.org> wrote: > > > > > > > > cpumasks are a pain, the above avoids allocating more of them. > > > > Indeed. > > > > > Yeah, so this could then be done by pointerifying ->cpus_allowed - more robust > > > than the wrappery, > > > > You mean: > > > > struct task_struct { > > cpumask_t cpus_allowed; > > cpumask_t *effective_cpus_allowed; > > };
Yeah. I'd name it a bit differently and constify the pointer to get type safety and to make sure the mask is never modified through the pointer:
struct task_struct { const cpumask_t *cpus_ptr; cpumask_t cpus_mask; };
( I'd drop the 'allowed' part, it's obvious enough what task->cpus_mask does, right? )
and upstream would essentially just do:
t->cpus_allowed_ptr = &t->cpus_allowed;
And -rt, when it wants to pin a task, would do:
t->cpus_allowed_ptr = &cpumask_of(task_cpu(p));
The rules are:
- Code that 'uses' ->cpus_allowed would use the pointer.
- Code that 'modifies' ->cpus_allowed would use the direct mask.
The upstream advantages are:
- The type separation of modifications from usage.
- Removal of wrappery.
- Maybe sometime in the future upstream would want to disable migration too ...
In fact -rt gains something too:
- With this scheme we would AFAICS get slightly more optimal code on -rt. (Because there's no __migration_disabled() branching anymore.)
- Plus all new code is automatically -rt ready - no need to maintain the wrappery space. Much less code path forking.
So as I see it it's win-win for both upstream and for -rt!
> > and make the scheduler use effective_cpus_allowed instead of cpus_allowed? Or > > what do you have in mind? > > That scheme is weird for nr_cpus_allowed. Not to mention that the > pointer to the integer is larger than the integer itself.
So in the new scheme I don't think nr_cpus_allowed would have to be wrapped at all: whenever the pointer (or mask) is changed in set_cpus_allowed_common() nr_cpus_allowed is recalculated as well - like today.
It should be self-maintaining. Am I missing something?
> I really prefer the current wrappers, they're trivial and consistent > with one another.
I think it's ugly wrappery and we can do better! ;-)
But of course if I cannot suggest a better alternative then it stands.
Thanks,
Ingo
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