Re: [PATCH 1/3] rcu: Replace magic number with meaningful constant in rcu_seq_done_exact()

[Joel Fernandes]

On 3/27/2025 1:09 PM, Paul E. McKenney wrote:
>>> In contrast, making the guard band a bit longer than it needs to be
>>> has little or no downside.
>> Making it 3 GP instead of 2 should be ok with me as long as we document it but at least it will not be a magic number based on an equation. I feel we should not put random magic numbers which is more dangerous since it is hard to explain (and hence debug — just my 2 cents).
> Apologies, I was getting ahead of us on this one.
> 
> That third (and maybe also a fourth) grace period becomes important if
> we start getting memory contention on rcu_state.gp_seq, in which case
> we would want the polled grace periods to look at the leaf rcu_node
> structure's gp_seq, which would introduce extra slop.

Makes sense.

> For now, for this patch, assuming you guys promise to remember this
> should such memory contention prove problematic:
> 
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Thanks! Will add the tag.

 - Joel