Le Fri, Nov 22, 2024 at 03:56:59PM +0100, Valentin Schneider a écrit :
> On 20/11/24 18:30, Frederic Weisbecker wrote:
> > Le Wed, Nov 20, 2024 at 06:10:43PM +0100, Valentin Schneider a écrit :
> >> On 20/11/24 15:23, Frederic Weisbecker wrote:
> >>
> >> > 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;
> >> > }
> >>
> >> Hmph, so that lets us leverage the cmpxchg for a !CT_STATE_KERNEL check,
> >> but we get an extra loop if the target CPU exits kernelspace not to
> >> userspace (e.g. vcpu or idle) in the meantime - not great, not terrible.
> >
> > The thing is, what you read with atomic_read() should be close to reality.
> > If it already is != CT_STATE_KERNEL then you're good (minus racy changes).
> > If it is CT_STATE_KERNEL then you still must do a failing cmpxchg() in any case,
> > at least to make sure you didn't miss a context tracking change. So the best
> > you can do is a bet.
> >
> >>
> >> At the cost of one extra bit for the CT_STATE area, with CT_STATE_KERNEL=1
> >> we could do:
> >>
> >> old = atomic_read(&ct->state);
> >> old &= ~CT_STATE_KERNEL;
> >
> > And perhaps also old |= CT_STATE_IDLE (I'm seeing the last patch now),
> > so you at least get a chance of making it right (only ~CT_STATE_KERNEL
> > will always fail) and CPUs usually spend most of their time idle.
> >
>
> I'm thinking with:
>
> CT_STATE_IDLE = 0,
> CT_STATE_USER = 1,
> CT_STATE_GUEST = 2,
> CT_STATE_KERNEL = 4, /* Keep that as a standalone bit */
Right!
>
> we can stick with old &= ~CT_STATE_KERNEL; and that'll let the cmpxchg
> succeed for any of IDLE/USER/GUEST.
Sure but if (old & CT_STATE_KERNEL), cmpxchg() will consistently fail.
But you can make a bet that it has switched to CT_STATE_IDLE between
the atomic_read() and the first atomic_cmpxchg(). This way you still have
a tiny chance to succeed.
That is:
old = atomic_read(&ct->state);
if (old & CT_STATE_KERNEl)
old |= CT_STATE_IDLE;
old &= ~CT_STATE_KERNEL;
do {
atomic_try_cmpxchg(...)
Hmm?
