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

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Message-ID: <Z0Oeme2yhxF_ArX0@pavilion.home>
Date: Sun, 24 Nov 2024 22:46:01 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [RFC PATCH v3 11/15] context-tracking: Introduce work deferral
 infrastructure

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?