Re: [PATCH v2 09/11] powerpc/smp: Optimize update_mask_by_l2

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Qian Cai <cai@redhat.com> [2020-10-07 09:05:42]:

Hi Qian,

Thanks for testing and reporting the failure.

> On Mon, 2020-09-21 at 15:26 +0530, Srikar Dronamraju wrote:
> > All threads of a SMT4 core can either be part of this CPU's l2-cache
> > mask or not related to this CPU l2-cache mask. Use this relation to
> > reduce the number of iterations needed to find all the CPUs that share
> > the same l2-cache.
> > 
> > Use a temporary mask to iterate through the CPUs that may share l2_cache
> > mask. Also instead of setting one CPU at a time into cpu_l2_cache_mask,
> > copy the SMT4/sub mask at one shot.
> > 
> ...
> >  static bool update_mask_by_l2(int cpu)
> >  {
> > +	struct cpumask *(*submask_fn)(int) = cpu_sibling_mask;
> >  	struct device_node *l2_cache, *np;
> > +	cpumask_var_t mask;
> >  	int i;
> >  
> >  	l2_cache = cpu_to_l2cache(cpu);
> > @@ -1240,22 +1264,37 @@ static bool update_mask_by_l2(int cpu)
> >  		return false;
> >  	}
> >  
> > -	cpumask_set_cpu(cpu, cpu_l2_cache_mask(cpu));
> > -	for_each_cpu_and(i, cpu_online_mask, cpu_cpu_mask(cpu)) {
> > +	alloc_cpumask_var_node(&mask, GFP_KERNEL, cpu_to_node(cpu));
> 
> Shouldn't this be GFP_ATOMIC? Otherwise, during the CPU hotplugging, we have,

Can you confirm if CONFIG_CPUMASK_OFFSTACK is enabled in your config?
Because if !CONFIG_CPUMASK_OFFSTACK, then alloc_cpumask_var_node would do
nothing but return true.

Regarding CONFIG_CPUMASK_OFFSTACK, not sure how much powerpc was tested
with that config enabled.

Please refer to
http://lore.kernel.org/lkml/87o8nv51bg.fsf@mpe.ellerman.id.au/t/#u
And we do have an issue to track the same.
https://github.com/linuxppc/issues/issues/321 for enabling/ testing /
verifying if CONFIG_CPUMASK_OFFSTACK works. I also dont see any
powerpc kconfig enabling this.

I do agree with your suggestion that we could substitute
GFP_ATOMIC/GFP_KERNEL.

> 
> (irqs were disabled in do_idle())
> 
> [  335.420001][    T0] BUG: sleeping function called from invalid context at mm/slab.h:494
> [  335.420003][    T0] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 0, name: swapper/88
> [  335.420005][    T0] no locks held by swapper/88/0.
> [  335.420007][    T0] irq event stamp: 18074448
> [  335.420015][    T0] hardirqs last  enabled at (18074447): [<c0000000001a2a7c>] tick_nohz_idle_enter+0x9c/0x110
> [  335.420019][    T0] hardirqs last disabled at (18074448): [<c000000000106798>] do_idle+0x138/0x3b0
> do_idle at kernel/sched/idle.c:253 (discriminator 1)
> [  335.420023][    T0] softirqs last  enabled at (18074440): [<c0000000000bbec4>] irq_enter_rcu+0x94/0xa0
> [  335.420026][    T0] softirqs last disabled at (18074439): [<c0000000000bbea0>] irq_enter_rcu+0x70/0xa0
> [  335.420030][    T0] CPU: 88 PID: 0 Comm: swapper/88 Tainted: G        W         5.9.0-rc8-next-20201007 #1
> [  335.420032][    T0] Call Trace:
> [  335.420037][    T0] [c00020000a4bfcf0] [c000000000649e98] dump_stack+0xec/0x144 (unreliable)
> [  335.420043][    T0] [c00020000a4bfd30] [c0000000000f6c34] ___might_sleep+0x2f4/0x310
> [  335.420048][    T0] [c00020000a4bfdb0] [c000000000354f94] slab_pre_alloc_hook.constprop.82+0x124/0x190
> [  335.420051][    T0] [c00020000a4bfe00] [c00000000035e9e8] __kmalloc_node+0x88/0x3a0
> slab_alloc_node at mm/slub.c:2817
> (inlined by) __kmalloc_node at mm/slub.c:4013
> [  335.420054][    T0] [c00020000a4bfe80] [c0000000006494d8] alloc_cpumask_var_node+0x38/0x80
> kmalloc_node at include/linux/slab.h:577
> (inlined by) alloc_cpumask_var_node at lib/cpumask.c:116
> [  335.420060][    T0] [c00020000a4bfef0] [c00000000003eedc] start_secondary+0x27c/0x800
> update_mask_by_l2 at arch/powerpc/kernel/smp.c:1267
> (inlined by) add_cpu_to_masks at arch/powerpc/kernel/smp.c:1387
> (inlined by) start_secondary at arch/powerpc/kernel/smp.c:1420
> [  335.420063][    T0] [c00020000a4bff90] [c00000000000c468] start_secondary_resume+0x10/0x14
> 
> > +	cpumask_and(mask, cpu_online_mask, cpu_cpu_mask(cpu));
> > +
> > +	if (has_big_cores)
> > +		submask_fn = cpu_smallcore_mask;
> > +
> > +	/* Update l2-cache mask with all the CPUs that are part of submask */
> > +	or_cpumasks_related(cpu, cpu, submask_fn, cpu_l2_cache_mask);
> > +
> > +	/* Skip all CPUs already part of current CPU l2-cache mask */
> > +	cpumask_andnot(mask, mask, cpu_l2_cache_mask(cpu));
> > +
> > +	for_each_cpu(i, mask) {
> >  		/*
> >  		 * when updating the marks the current CPU has not been marked
> >  		 * online, but we need to update the cache masks
> >  		 */
> >  		np = cpu_to_l2cache(i);
> > -		if (!np)
> > -			continue;
> >  
> > -		if (np == l2_cache)
> > -			set_cpus_related(cpu, i, cpu_l2_cache_mask);
> > +		/* Skip all CPUs already part of current CPU l2-cache */
> > +		if (np == l2_cache) {
> > +			or_cpumasks_related(cpu, i, submask_fn,
> > cpu_l2_cache_mask);
> > +			cpumask_andnot(mask, mask, submask_fn(i));
> > +		} else {
> > +			cpumask_andnot(mask, mask, cpu_l2_cache_mask(i));
> > +		}
> >  
> >  		of_node_put(np);
> >  	}
> >  	of_node_put(l2_cache);
> > +	free_cpumask_var(mask);
> >  
> >  	return true;
> >  }
> 

-- 
Thanks and Regards
Srikar Dronamraju