Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()

[Valentin Schneider <valentin.schneider@arm.com>]

On 10/08/21 20:49, Boqun Feng wrote:
> On Sun, Aug 08, 2021 at 05:15:20PM +0100, Valentin Schneider wrote:
>> On 07/08/21 03:42, Mike Galbraith wrote:
>> > On Sat, 2021-08-07 at 01:58 +0100, Valentin Schneider wrote:
>> >>
>> >> +static inline bool is_pcpu_safe(void)
>> >
>> > Nit: seems odd to avoid spelling it out to save two characters, percpu
>> > is word like, rolls off the ole tongue better than p-c-p-u.
>> >
>> >       -Mike
>> 
>> True. A quick grep says both versions are used, though "percpu" wins by
>> about a factor of 2. I'll tweak that for a v3.
>
> I wonder why is_percpu_safe() is the correct name. The safety of
> accesses to percpu variables means two things to me:
>
> a)	The thread cannot migrate to other CPU in the middle of
> 	accessing a percpu variable, in other words, the following
> 	cannot happen:
>
> 	{ percpu variable X is 0 on CPU 0 and 2 on CPU 1
> 	CPU 0				CPU 1
> 	========			=========
> 	<in thread A>
> 	__this_cpu_inc(X);
> 	  tmp = X; // tmp is 0
> 	  <preempted>
> 	  <migrate to CPU 1>
> 	  				// continue __this_cpu_inc(X);
> 					X = tmp + 1; // CPU 0 miss this
> 						     // increment (this
> 						     // may be OK), and
> 						     // CPU 1's X got
> 						     // corrupted.
>
> b)	The accesses to a percpu variable are exclusive, i.e. no
> 	interrupt or preemption can happen in the middle of accessing,
> 	in other words, the following cannot happen:
>
> 	{ percpu variable X is 0 on CPU 0 }
> 	CPU 0
> 	========
> 	<in thread A>
> 	__this_cpu_inc(X);
> 	  tmp = X; // tmp is 0
> 	  <preempted>
> 	  <in other thread>
> 	  this_cpu_inc(X); // X is 1 afterwards.
> 	  <back to thread A>
> 	  X = tmp + 1; // X is 1, and we have a race condition.
>
> And the is_p{er}cpu_safe() only detects the first, and it doesn't mean
> totally safe for percpu accesses.
>

Right. I do briefly point this out in the changelog (the bit about
"acquiring a sleepable lock if relevant"), but that doesn't do much to
clarify the helper name itself.

> Maybe we can implement a migratable()? Although not sure it's a English
> word.
>

Funnily enough that is exactly how I named the thing in my initial draft,
but then I somehow convinced myself that tailoring the name to per-CPU
accesses would make its intent clearer.

I think you're right that "migratable()" is less confusing at the end of
the day. Oh well, so much for overthinking the naming problem :-)

> Regards,
> Boqun