Re: [RFC PATCH 11/14] context-tracking: Introduce work deferral infrastructure

[Valentin Schneider]

On 06/07/23 00:23, Frederic Weisbecker wrote:
> Le Wed, Jul 05, 2023 at 07:12:53PM +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_work;
>> +	bool ret = false;
>> +
>> +	preempt_disable();
>> +
>> +	old_work = atomic_read(&ct->work);
>> +	/*
>> +	 * Try setting the work until either
>> +	 * - the target CPU no longer accepts any more deferred work
>> +	 * - the work has been set
>> +	 */
>> +	while (!(old_work & CONTEXT_WORK_DISABLED) && !ret)
>
> Isn't there a race here where you may have missed a CPU that just entered in
> user and you eventually disturb it?
>

Yes, unfortunately.

>> +		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.

> 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?