Messages in this thread | | | Date | Fri, 15 Oct 2021 15:26:00 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/apic: reduce cache line misses in __x2apic_send_IPI_mask() |
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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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