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Date:   Thu, 6 Jul 2023 13:40:14 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Valentin Schneider <vschneid@redhat.com>
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Subject: Re: [RFC PATCH 11/14] context-tracking: Introduce work deferral
 infrastructure
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References: <20230705181256.3539027-1-vschneid@redhat.com>
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 <ZKXtfWZiM66dK5xC@localhost.localdomain>
 <xhsmhttuhuvix.mognet@vschneid.remote.csb>
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On Thu, Jul 06, 2023 at 12:30:46PM +0100, Valentin Schneider wrote:
> >> +		ret = atomic_try_cmpxchg(&ct->work, &old_work, old_work | work);
> >> +
> >> +	preempt_enable();
> >> +	return ret;
> >> +}
> > [...]
> >> @@ -100,14 +158,19 @@ static noinstr void ct_kernel_exit_state(int offset)
> >>   */
> >>  static noinstr void ct_kernel_enter_state(int offset)
> >>  {
> >> +	struct context_tracking *ct = this_cpu_ptr(&context_tracking);
> >>      int seq;
> >> +	unsigned int work;
> >>
> >> +	work = ct_work_fetch(ct);
> >
> > So this adds another fully ordered operation on user <-> kernel transition.
> > How many such IPIs can we expect?
> >
> 
> Despite having spent quite a lot of time on that question, I think I still
> only have a hunch.
> 
> Poking around RHEL systems, I'd say 99% of the problematic IPIs are
> instruction patching and TLB flushes.
> 
> Staring at the code, there's quite a lot of smp_calls for which it's hard
> to say whether the target CPUs can actually be isolated or not (e.g. the
> CPU comes from a cpumask shoved in a struct that was built using data from
> another struct of uncertain origins), but then again some of them don't
> need to hook into context_tracking.
> 
> Long story short: I /think/ we can consider that number to be fairly small,
> but there could be more lurking in the shadows.

I guess it will still be time to reconsider the design if we ever reach such size.

> 
> > If this is just about a dozen, can we stuff them in the state like in the
> > following? We can potentially add more of them especially on 64 bits we could
> > afford 30 different works, this is just shrinking the RCU extended quiescent
> > state counter space. Worst case that can happen is that RCU misses 65535
> > idle/user <-> kernel transitions and delays a grace period...
> >
> 
> I'm trying to grok how this impacts RCU, IIUC most of RCU mostly cares about the
> even/odd-ness of the thing, and rcu_gp_fqs() cares about the actual value
> but only to check if it has changed over time (rcu_dynticks_in_eqs_since()
> only does a !=).
> 
> I'm rephrasing here to make sure I get it - is it then that the worst case
> here is 2^(dynticks_counter_size) transitions happen between saving the
> dynticks snapshot and checking it again, so RCU waits some more?

That's my understanding as well but I have to defer on Paul to make sure I'm
not overlooking something.

Thanks.