Re: [PATCH 1/3] powerpc/smp: Fix a crash while booting kvm guest with nr_cpus=2
From: Gautham R Shenoy <hidden>
Date: 2021-08-23 06:12:29
On Sat, Aug 21, 2021 at 02:54:17PM +0530, Srikar Dronamraju wrote:
quoted hunk ↗ jump to hunk
Aneesh reported a crash with a fairly recent upstream kernel when booting kernel whose commandline was appended with nr_cpus=2 1:mon> e cpu 0x1: Vector: 300 (Data Access) at [c000000008a67bd0] pc: c00000000002557c: cpu_to_chip_id+0x3c/0x100 lr: c000000000058380: start_secondary+0x460/0xb00 sp: c000000008a67e70 msr: 8000000000001033 dar: 10 dsisr: 80000 current = 0xc00000000891bb00 paca = 0xc0000018ff981f80 irqmask: 0x03 irq_happened: 0x01 pid = 0, comm = swapper/1 Linux version 5.13.0-rc3-15704-ga050a6d2b7e8 (kvaneesh@ltc-boston8) (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #433 SMP Tue May 25 02:38:49 CDT 2021 1:mon> t [link register ] c000000000058380 start_secondary+0x460/0xb00 [c000000008a67e70] c000000008a67eb0 (unreliable) [c000000008a67eb0] c0000000000589d4 start_secondary+0xab4/0xb00 [c000000008a67f90] c00000000000c654 start_secondary_prolog+0x10/0x14 Current code assumes that num_possible_cpus() is always greater than threads_per_core. However this may not be true when using nr_cpus=2 or similar options. Handle the case where num_possible_cpus is smaller than threads_per_core. Cc: linuxppc-dev@lists.ozlabs.org Cc: Aneesh Kumar K.V <redacted> Cc: Nathan Lynch <redacted> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Valentin Schneider <redacted> Cc: Gautham R Shenoy <redacted> Cc: Vincent Guittot <vincent.guittot@linaro.org> Fixes: c1e53367dab1 ("powerpc/smp: Cache CPU to chip lookup") Reported-by: Aneesh Kumar K.V <redacted> Debugged-by: Michael Ellerman [off-list ref] Signed-off-by: Srikar Dronamraju <redacted> --- arch/powerpc/kernel/smp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c index 6c6e4d934d86..3d6874fe1937 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c@@ -1074,7 +1074,7 @@ void __init smp_prepare_cpus(unsigned int max_cpus) } if (cpu_to_chip_id(boot_cpuid) != -1) { - int idx = num_possible_cpus() / threads_per_core; + int idx = max((int)num_possible_cpus() / threads_per_core, 1);
I think this code was assuming that num_possible_cpus() is a multiple of threads_per_core. So, on a system with threads_per_core=8, if we pass nr_cpus=10, we will still get idx=1. Thus, we will allocate only one entry in chip_id_lookup_table[] even though there are two cores and chip_id_lookup_table[] is expected to have one entry per core. Is this a valid scenario ? If yes, should we use idx = DIV_ROUND_UP(num_possible_cpus, threads_per_core); ?
/* * All threads of a core will all belong to the same core, -- 2.18.2
-- Thanks and Regards gautham.