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? 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. > > 2) some archs do not support 'nr_cpus=1', and have to rely on > 'max_cpus=1', so num_possible_cpus isn't changed, and kernel just boots > with single online cpu. That causes trouble because blk-mq limits single > queue. > > Documentation/admin-guide/kdump/kdump.rst > > Thanks, > Ming >
