Re: [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group

[Reinette Chatre]

Hi Valentin,

On 12/16/2020 9:41 AM, Valentin Schneider wrote:
> On 14/12/20 18:41, Reinette Chatre wrote:
> > > > -	return ret;
> > > > +
> > > > +	/*
> > > > +	 * By now, the task's closid and rmid are set. If the task is current
> > > > +	 * on a CPU, the PQR_ASSOC MSR needs to be updated to make the resource
> > > > +	 * group go into effect. If the task is not current, the MSR will be
> > > > +	 * updated when the task is scheduled in.
> > > > +	 */
> > > > +	update_task_closid_rmid(tsk);
> > >
> > > We need the above writes to be compile-ordered before the IPI is sent.
> > > There *is* a preempt_disable() down in smp_call_function_single() that
> > > gives us the required barrier(), can we deem that sufficient or would we
> > > want one before update_task_closid_rmid() for the sake of clarity?
> >
> > Apologies, it is not clear to me why the preempt_disable() would be
> > insufficient. If it is not then there may be a few other areas (where
> > resctrl calls smp_call_function_xxx()) that needs to be re-evaluated.
>
> So that's part paranoia and part nonsense from my end - the contents of
> smp_call() shouldn't matter here.
>
> If we distill the code to:
>
>    tsk->closid = x;
>
>    if (task_curr(tsk))
>        smp_call(...);
>
> It is somewhat far fetched, but AFAICT this can be compiled as:
>
>    if (task_curr(tsk))
>        tsk->closid = x;
>        smp_call(...);
>    else
>        tsk->closid = x;
>
> IOW, there could be a sequence where the closid write is ordered *after*
> the task_curr() read.

Could you please elaborate why it would be an issue is the closid write 
is ordered after the task_curr() read? task_curr() does not depend on 
the closid.

> With
>
>    tsk->closid = x;
>
>    barrier();
>
>    if (task_curr(tsk))
>        smp_call(...);
>
> that explicitely cannot happen.


Reinette