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SubjectRe: [PATCH] x86/apic: reduce cache line misses in __x2apic_send_IPI_mask()
On Wed, Oct 13, 2021 at 11:02:46AM -0700, Eric Dumazet wrote:
> On Tue, Oct 12, 2021 at 5:46 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> > I'm really conflicted about this. On the one hand, yes absolutely. On
> > the other hand, urgh, code ugly :-)
>
> That was indeed some ugly hack.
>
> I cooked this more generic patch instead, I am currently testing it.
> (generic as : we no longer disable hard irqs, regardless of some CONFIG option )
>
> diff --git a/arch/x86/kernel/apic/x2apic_cluster.c
> b/arch/x86/kernel/apic/x2apic_cluster.c
> index e696e22d0531976f7cba72ed17443592eac72c13..7ad81467ce33349dee1ceaf0cefc8375d60213f6
> 100644
> --- a/arch/x86/kernel/apic/x2apic_cluster.c
> +++ b/arch/x86/kernel/apic/x2apic_cluster.c
> @@ -22,7 +22,10 @@ struct cluster_mask {
> */
> static u32 *x86_cpu_to_logical_apicid __read_mostly;
>
> -static DEFINE_PER_CPU(cpumask_var_t, ipi_mask);

This might maybe do with a comment explaining where the 3 comes from.
Also see below.

> +#define IPI_NEST_MAX 3
> +static DEFINE_PER_CPU(cpumask_var_t, ipi_mask[IPI_NEST_MAX]);
> +static DEFINE_PER_CPU(int, ipi_nest_level);
> +
> static DEFINE_PER_CPU_READ_MOSTLY(struct cluster_mask *, cluster_masks);
> static struct cluster_mask *cluster_hotplug_mask;
>
> @@ -45,14 +48,18 @@ __x2apic_send_IPI_mask(const struct cpumask *mask,
> int vector, int apic_dest)
> {
> unsigned int cpu, clustercpu;
> struct cpumask *tmpmsk;
> - unsigned long flags;
> + int nest_level;
> u32 dest;
>
> /* x2apic MSRs are special and need a special fence: */
> weak_wrmsr_fence();
> - local_irq_save(flags);
>
> - tmpmsk = this_cpu_cpumask_var_ptr(ipi_mask);
> + preempt_disable();
> + nest_level = this_cpu_inc_return(ipi_nest_level) - 1;
> + if (WARN_ON_ONCE(nest_level >= IPI_NEST_MAX))
> + goto end;

So this matches the: task, softirq, irq nesting and realistically won't
trigger I suppose, but that WARN is not giving me warm and fuzzies, just
not sending the IPI is terrible behaviour if we ever do hit this.

I think I would prefer to trip x2apic_send_IPI_all() over sending too
few IPIs.

That *might* in some distant future kill some NOHZ_FULL userspace, but
at least it won't make the system grind to a halt as a missing IPI can.

Thomas, any opinions there?

> +
> + tmpmsk = this_cpu_cpumask_var_ptr(ipi_mask[nest_level]);
> cpumask_copy(tmpmsk, mask);
> /* If IPI should not be sent to self, clear current CPU */
> if (apic_dest != APIC_DEST_ALLINC)
> @@ -74,7 +81,9 @@ __x2apic_send_IPI_mask(const struct cpumask *mask,
> int vector, int apic_dest)
> cpumask_andnot(tmpmsk, tmpmsk, &cmsk->mask);
> }
>
> - local_irq_restore(flags);
> +end:
> + this_cpu_dec(ipi_nest_level);
> + preempt_enable();
> }
>
> static void x2apic_send_IPI_mask(const struct cpumask *mask, int vector)

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