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

[David Hildenbrand (Arm)]

On 6/29/26 18:54, Andi Kleen wrote:
>> 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)

We discussed that a bit already. Hypervisors make it fairly unpredictable how
long you would actually have to spin.

-- 
Cheers,

David