On Thu, Mar 9, 2023 at 3:53 PM David Matlack <dmatlack@xxxxxxxxxx> wrote: > > On Mon, Mar 06, 2023 at 02:41:12PM -0800, Vipin Sharma wrote: > > Create a global counter for total number of pages available > > in MMU page caches across all VMs. Add mmu_shadow_page_cache > > pages to this counter. > > I think I prefer counting the objects on-demand in mmu_shrink_count(), > instead of keeping track of the count. Keeping track of the count adds > complexity to the topup/alloc paths for the sole benefit of the > shrinker. I'd rather contain that complexity to the shrinker code unless > there is a compelling reason to optimize mmu_shrink_count(). > > IIRC we discussed this at one point. Was there a reason to take this > approach that I'm just forgetting? To count on demand, we first need to lock on kvm_lock and then for each VMs iterate through each vCPU, take a lock, and sum the objects count in caches. When the NUMA support will be introduced in this series then it means we have to iterate even more caches. We can't/shouldn't use mutex_trylock() as it will not give the correct picture and when shrink_scan is called count can be totally different. scan_count() API comment says to not do any deadlock check (I don't know what does that mean) and percpu_counter is very fast when we are adding/subtracting pages so the effect of using it to keep global count is very minimal. Since, there is not much impact to using percpu_count compared to previous one, we ended our discussion with keeping this per cpu counter.
