Re: [PATCH 0/5] Remove onstack cpumask var usage

[Yury Norov <yury.norov@gmail.com>]

On Fri, Apr 19, 2024 at 05:26:34PM +0800, Dawei Li wrote:
> Hi Sam,
> 
> Thanks for the review.
> 
> On Fri, Apr 19, 2024 at 07:13:50AM +0200, Sam Ravnborg wrote:
> > Hi Dawei,
> > 
> > On Thu, Apr 18, 2024 at 06:49:44PM +0800, Dawei Li wrote:
> > > Hi,
> > > 
> > > This series aims at removing on-stack cpumask var usage for sparc arch.
> > > 
> > > Generally it's preferable to avoid placing cpumasks on the stack, as
> > > for large values of NR_CPUS these can consume significant amounts of
> > > stack space and make stack overflows more likely.
> > 
> > Took a quick look at the patches, looks good except the one the bot
> > already complained about.
> 
> I will fix this building warning in respinning.
> 
> > A quick grep shows a few more cases where we have an on-stack cpumask
> > in sparc code.
> > 
> > kernel/ds.c:    cpumask_t mask;
> 
> About this case, it's kinda tricky for:
> - dr_cpu_data() returns void, so alloc_cpumask_var() is no go.
> 
> - No idea of the calling context of dr_cpu_data(). IIUC,
>   dr_cpu_data()
>   ->dr_cpu_configure()  
>    ->kzalloc(resp_len, GFP_KERNEL)
>   So I guess it's in process context?
>   If consumption above is OK, a simple but _ugly_ solution could be:
> 
> diff --git a/arch/sparc/kernel/ds.c b/arch/sparc/kernel/ds.c
> index ffdc15588ac2..c9e4ebdccf49 100644
> --- a/arch/sparc/kernel/ds.c
> +++ b/arch/sparc/kernel/ds.c
> @@ -634,7 +634,8 @@ static void dr_cpu_data(struct ds_info *dp, struct ds_cap_state *cp, void *buf,
>         struct dr_cpu_tag *tag = (struct dr_cpu_tag *) (data + 1);
>         u32 *cpu_list = (u32 *) (tag + 1);
>         u64 req_num = tag->req_num;
> -       cpumask_t mask;
> +       static DEFINE_MUTEX(mask_lock);
> +       static cpumask_t mask;
>         unsigned int i;
>         int err;
> 
> @@ -651,6 +652,8 @@ static void dr_cpu_data(struct ds_info *dp, struct ds_cap_state *cp, void *buf,
> 
>         purge_dups(cpu_list, tag->num_records);
> 
> +       mutex_lock(&mask_lock);
> +
>         cpumask_clear(&mask);
>         for (i = 0; i < tag->num_records; i++) {
>                 if (cpu_list[i] == CPU_SENTINEL)
> @@ -665,6 +668,8 @@ static void dr_cpu_data(struct ds_info *dp, struct ds_cap_state *cp, void *buf,
>         else
>                 err = dr_cpu_unconfigure(dp, cp, req_num, &mask);
> 
> +       mutex_unlock(&mask_lock);
> +
>         if (err)
>                 dr_cpu_send_error(dp, cp, data);
>  }
> 
> How does it sound to you?
> 
> > kernel/leon_kernel.c:   cpumask_t mask;
> 
> It's in irqchip::irq_set_affinity(), which is in atomic context(raw spinlock(s) held),
> so dynamic allocation is not a good idea.
> 
> My proposal(*untested*) is somewhat complicated for it introduces a new helper.
> 
> diff --git a/arch/sparc/kernel/leon_kernel.c b/arch/sparc/kernel/leon_kernel.c
> index 4c61da491fee..6eced7acb8bc 100644
> --- a/arch/sparc/kernel/leon_kernel.c
> +++ b/arch/sparc/kernel/leon_kernel.c
> @@ -104,15 +104,25 @@ unsigned long leon_get_irqmask(unsigned int irq)
>  }
> 
>  #ifdef CONFIG_SMP
> +
> +static bool cpumask_include(const struct cpumask *srcp1, const struct cpumask *srcp2)

Don't steal the other's subsystems prefixes.

> +{
> +       unsigned int cpu;
> +
> +       for_each_cpu(cpu, srcp2) {
> +               if (!cpumask_test_cpu(cpu, srcp1))
> +                       return false;
> +       }
> +
> +       return true;
> +}

We've got cpumask_subset() for this. 

>  static int irq_choose_cpu(const struct cpumask *affinity)
>  {
> -       cpumask_t mask;
> +       unsigned int cpu = cpumask_first_and(affinity, cpu_online_mask);
> 
> -       cpumask_and(&mask, cpu_online_mask, affinity);
> -       if (cpumask_equal(&mask, cpu_online_mask) || cpumask_empty(&mask))
> -               return boot_cpu_id;
> -       else
> -               return cpumask_first(&mask);
> +       return cpumask_include(affinity, cpu_online_mask) || cpu >= nr_cpu_ids ?
> +              boot_cpu_id : cpu;
>  }
>  #else
>  #define irq_choose_cpu(affinity) boot_cpu_id
> 
> Is it OK?
> 
> [cc Yury for bitmap API]
> 
> > kernel/leon_smp.c:static void leon_cross_call(void *func, cpumask_t mask, unsigned long arg1,
> > kernel/sun4d_smp.c:static void sun4d_cross_call(void *func, cpumask_t mask, unsigned long arg1,
> 
> Actually I am awared of existence of (at least some of) them, but so far I
> have not found a _proper_ way of dealing with them(especially for case of
> ds.c).
> 
> Please lemme dig into it.
> 
> Thanks,
> 
>     Dawei
> 
> > 
> > Do you plan to look at the other on-stack users too?
> > It would be nice to see them all gone in one patch-set.
> > 
> > 	Sam
> >