Re: [PATCH] x86/resctrl: Fix WARN in get_domain_from_cpu()

[James Morse]

Hi Tony,

On 21/02/2024 00:34, Tony Luck wrote:
> reset_all_ctrls() and resctrl_arch_update_domains() use
> on_each_cpu_mask() to call rdt_ctrl_update() on potentially
> one CPU from each domain.
> 
> But this means rdt_ctrl_update() needs to figure out which domain
> to apply changes to. Doing so requires a search of all domains
> in a resource, which can only be done safely if cpus_lock is
> held. Both callers do hold this lock, but there isn't a way
> for a function called on another CPU via IPI to verify this.
> 
> Fix by adding the target domain to the msr_param structure and
> calling for each domain separately using smp_call_function_single()

Cunning - this trades the memory allocation for multiple IPI. I think this is much better
for the case where only the local domains configuration is modified.

With the double-IPI when both CDP configurations are changed fixed:
Reviewed-by: James Morse <james.morse@xxxxxxx>

I think we should rip out the false positive check, I'll post a patch to do that.
I'll double check this was the only IPI path, if so its safe again after this patch and we
can add lockdep_assert_cpus_held(). If anyone ever hits this during a bisect its should be
clear(er).


Thanks,

James