Re: [RFC PATCH v3 11/15] context-tracking: Introduce work deferral infrastructure

[Frederic Weisbecker]

Le Wed, Nov 20, 2024 at 11:54:36AM +0100, Frederic Weisbecker a écrit :
> Le Tue, Nov 19, 2024 at 04:34:58PM +0100, Valentin Schneider a écrit :
> > +bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> > +{
> > +   struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> > +   unsigned int old;
> > +   bool ret = false;
> > +
> > +   preempt_disable();
> > +
> > +   old = atomic_read(&ct->state);
> > +   /*
> > +    * Try setting the work until either
> > +    * - the target CPU has entered kernelspace
> > +    * - the work has been set
> > +    */
> > +   do {
> > +           ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << 
> > CT_WORK_START));
> > +   } while (!ret && ((old & CT_STATE_MASK) != CT_STATE_KERNEL));
> > +
> > +   preempt_enable();
> > +   return ret;
> 
> Does it ignore the IPI even if:
> 
>      (ret && (old & CT_STATE_MASK) == CT_STATE_KERNEL))
> 
> ?
> 
> And what about CT_STATE_IDLE?
> 
> Is the work ignored in those two cases?
> 
> But would it be cleaner to never set the work if the target is elsewhere
> than CT_STATE_USER. So you don't need to clear the work on kernel exit
> but rather on kernel entry.
> 
> That is:
> 
> bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> {
>       struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
>       unsigned int old;
>       bool ret = false;
> 
>       preempt_disable();
> 
>       old = atomic_read(&ct->state);
> 
>       /* Start with our best wishes */
>       old &= ~CT_STATE_MASK;
>       old |= CT_STATE_USER
> 
>       /*
>        * Try setting the work until either
>        * - the target CPU has exited userspace
>        * - the work has been set
>        */
>       do {
>               ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << 
> CT_WORK_START));
>       } while (!ret && ((old & CT_STATE_MASK) == CT_STATE_USER));
> 
>       preempt_enable();
> 
>       return ret;
> }

Ah but there is CT_STATE_GUEST and I see the last patch also applies that to
CT_STATE_IDLE.

So that could be:

bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
{
        struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
        unsigned int old;
        bool ret = false;

        preempt_disable();

        old = atomic_read(&ct->state);

        /* CT_STATE_IDLE can be added to last patch here */
        if (!(old & (CT_STATE_USER | CT_STATE_GUEST))) {
                old &= ~CT_STATE_MASK;
                old |= CT_STATE_USER;
        }

        /*
         * Try setting the work until either
         * - the target CPU has exited userspace / guest
         * - the work has been set
         */
        do {
                ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << 
CT_WORK_START));
        } while (!ret && old & (CT_STATE_USER | CT_STATE_GUEST));

        preempt_enable();

        return ret;
}

Thanks.