On Thu, Aug 10, 2023 at 09:18:27AM +0800, Baoquan He wrote: > On 08/10/23 at 08:09am, Ming Lei wrote: > > On Wed, Aug 09, 2023 at 03:44:01PM +0200, Christoph Hellwig wrote: > > > I'm starting to sound like a broken record, but we can't just do random > > > is_kdump checks, and it's not going to get better by resending it again and > > > again. If kdump kernels limit the number of possible CPUs, it needs to > > > reflected in cpu_possible_map and we need to use that information. > > > > > > > Can you look at previous kdump/arch guys' comment about kdump usage & > > num_possible_cpus? > > > > https://lore.kernel.org/linux-block/CAF+s44RuqswbosY9kMDx35crviQnxOeuvgNsuE75Bb0Y2Jg2uw@xxxxxxxxxxxxxx/ > > https://lore.kernel.org/linux-block/ZKz912KyFQ7q9qwL@MiWiFi-R3L-srv/ > > > > The point is that kdump kernels does not limit the number of possible CPUs. > > > > 1) some archs support 'nr_cpus=1' for kdump kernel, which is fine, since > > num_possible_cpus becomes 1. > > Yes, "nr_cpus=" is strongly suggested in kdump kernel because "nr_cpus=" > limits the possible cpu numbers, while "maxcpuss=" only limits the cpu > number which can be brought up during bootup. We noticed this diference > because a large number of possible cpus will cost more memory in kdump > kernel. e.g percpu initialization, even though kdump kernel have set > "maxcpus=1". > > Currently x86 and arm64 all support "nr_cpus=". Pingfan ever spent much > effort to make patches to add "nr_cpus=" support to ppc64, seems ppc64 > dev and maintainers do not care about it. Finally the patches are not > accepted, and the work is not continued. > > Now, I am wondering what is the barrier to add "nr_cpus=" to power ach. > Can we reconsider adding 'nr_cpus=' to power arch since real issue > occurred in kdump kernel? If 'nr_cpus=' can be supported on ppc64, this patchset isn't needed. > > As for this patchset, it can be accpeted so that no failure in kdump > kernel is seen on ARCHes w/o "nr_cpus=" support? My personal opinion. IMO 'nr_cpus=' support should be preferred, given it is annoying to maintain two kinds of implementation for kdump kernel from driver viewpoint. I guess kdump things can be simplified too with supporting 'nr_cpus=' only. thanks, Ming
