Re: [PATCH 0/2] mm: memory-failure: fix HWPoison flag race with non-atomic page flag ops

[Andi Kleen]

> However, this was a basic test, when allocating 4k pages. With 2M hugepages:
> 
>                                    insns/iter    cycles/iter
>      -------------------------------------------------------
>        base                       20758 +/- 12.5   191208 +/-1946.6
>        rcu                        20785 +/-  3.7   197108 +/- 132.1
>        atomic                     20727 +/-  6.4   204591 +/- 160.2
> 
>        rcu vs base                  +27 (+0.13%)     +5900 (+3.09%)
>        atomic vs base               -31 (-0.15%)    +13383 (+7.00%)
> 
> and even with THP:
> 
>                                    insns/iter    cycles/iter
>      -------------------------------------------------------
>        base                       27220 +/-  2.8   192151 +/- 483.3
>        rcu                        27248 +/- 30.1   194159 +/-2746.6
>        atomic                     27186 +/-  3.2   200526 +/- 746.2
> 
>        rcu vs base                  +28 (+0.10%)     +2008 (+1.04%)
>        atomic vs base               -34 (-0.12%)     +8374 (+4.36%)
> 
> 
> needs more thought.

Well the alternative is to not bother with RCU, but just wait a bit and
check if the bit stuck and repeat if needed. While that could in theory
livelock it is extremely unlikely (especially if you add a bit of randomization 
to the sleep)

-Andi