Re: [PATCH 8/8] shmem,percpu_counter: add _limited_add(fbc, limit, amount)

[Hugh Dickins <hughd@google.com>]

On Fri, 6 Oct 2023, Dennis Zhou wrote:
> 
> Sorry for the late chime in. I'm traveling right now.

No problem at all, thanks for looking.

> 
> I haven't been super happy lately with percpu_counter as it has had a
> few corner cases such as the cpu_dying_mask fiasco which I thought we
> fixed with a series from tglx [1]. If not I can resurrect it and pull
> it.
> 
> I feel like percpu_counter is deviating from its original intended
> usecase which, from my perspective, was a thin wrapper around a percpu
> variable. At this point we seem to be bolting onto percpu_counter
> instead of giving it a clear focus for what it's supposed to do well.
> I think I understand the use case, and ultimately it's kind of the
> duality where I think it was xfs is using percpu_counters where it must
> be > 0 for the value to make sense and there was a race condition with
> cpu dying [2].
> 
> At this point, I think it's probably better to wholy think about the
> lower bound and upper bound problem of percpu_counter wrt the # of
> online cpus.
> 
> Thanks,
> Dennis
> 
> [1] https://lore.kernel.org/lkml/20230414162755.281993820@linutronix.de/
> [2] https://lore.kernel.org/lkml/20230406015629.1804722-1-yebin@huaweicloud.com/

Thanks for the links.  I can see that the current cpu_dying situation
is not ideal, but don't see any need to get any deeper into that for
percpu_counter_limited_add(): I did consider an update to remove its
use of cpu_dying_mask, but that just seemed wrong - it should do the
same as is currently done in __percpu_counter_sum(), then be updated
along with that when cpu_dying is sorted, by tglx's series or otherwise.

I don't think I agree with you about percpu_counter deviating from its
original intended usecase; but I haven't studied the history to see what
that initial usecase was.  Whatever, we've had percpu_counter_add() and
percpu_counter_compare() for many years, and percpu_counter_limited_add()
is just an atomic combination of those: I don't see it as deviating at all.

Hugh