On Fri, Nov 27, 2020 at 01:23:06PM +0000, Qais Yousef wrote: > On 11/24/20 15:50, Will Deacon wrote: > > When exec'ing a 32-bit task on a system with mismatched support for > > 32-bit EL0, try to ensure that it starts life on a CPU that can actually > > run it. > > > > Signed-off-by: Will Deacon <will@xxxxxxxxxx> > > --- > > arch/arm64/kernel/process.c | 42 ++++++++++++++++++++++++++++++++++++- > > 1 file changed, 41 insertions(+), 1 deletion(-) > > > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > > index 1540ab0fbf23..72116b0c7c73 100644 > > --- a/arch/arm64/kernel/process.c > > +++ b/arch/arm64/kernel/process.c > > @@ -31,6 +31,7 @@ > > #include <linux/interrupt.h> > > #include <linux/init.h> > > #include <linux/cpu.h> > > +#include <linux/cpuset.h> > > #include <linux/elfcore.h> > > #include <linux/pm.h> > > #include <linux/tick.h> > > @@ -625,6 +626,45 @@ unsigned long arch_align_stack(unsigned long sp) > > return sp & ~0xf; > > } > > > > +static void adjust_compat_task_affinity(struct task_struct *p) > > +{ > > + cpumask_var_t cpuset_mask; > > + const struct cpumask *possible_mask = system_32bit_el0_cpumask(); > > + const struct cpumask *newmask = possible_mask; > > + > > + /* > > + * Restrict the CPU affinity mask for a 32-bit task so that it contains > > + * only the 32-bit-capable subset of its original CPU mask. If this is > > + * empty, then try again with the cpuset allowed mask. If that fails, > > + * forcefully override it with the set of all 32-bit-capable CPUs that > > + * we know about. > > + * > > + * From the perspective of the task, this looks similar to what would > > + * happen if the 64-bit-only CPUs were hot-unplugged at the point of > > + * execve(). > > + */ > > + if (!restrict_cpus_allowed_ptr(p, possible_mask)) > > + goto out; > > + > > + if (alloc_cpumask_var(&cpuset_mask, GFP_KERNEL)) { > > + cpuset_cpus_allowed(p, cpuset_mask); > > + if (cpumask_and(cpuset_mask, cpuset_mask, possible_mask)) { > > + newmask = cpuset_mask; > > + goto out_set_mask; > > + } > > + } > > Wouldn't it be better to move this logic to restrict_cpus_allowed_ptr()? > I think it should always take cpusets into account and it's not special to > this particular handling here, no? I did actually try this but didn't pursue it further because I was worried that I was putting too much of the "can't run a 32-bit task on a 64-bit-only CPU" logic into what would otherwise be a potentially useful library function if/when other architectures want something similar. But I'll have another look because there were a couple of ideas I didn't try out. > > + if (printk_ratelimit()) { > > + printk_deferred("Overriding affinity for 32-bit process %d (%s) to CPUs %*pbl\n", > > + task_pid_nr(p), p->comm, cpumask_pr_args(newmask)); > > + } > > We have 2 cases where the affinity could have been overridden but we won't > print anything: > > 1. restrict_cpus_allowed_ptr() > 2. intersection of cpuset_mask and possible mask drops some cpus. > > Shouldn't we print something in these cases too? I don't think so: in these cases we've found a subset of CPUs that we can run on, and so there's no need to warn. Nothing says we _have_ to use all the CPUs available to us. The case where we override the affinity mask altogether, however, does warrant a warning. This is very similar to the hotplug behaviour in select_fallback_rq(). Will