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x86/apic: Reduce cache line misses in __x2apic_send_IPI_mask()

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Using per-cpu storage for @x86_cpu_to_logical_apicid is not optimal.

Broadcast IPI will need at least one cache line per cpu to access this
field.

__x2apic_send_IPI_mask() is using standard bitmask operators.

By converting x86_cpu_to_logical_apicid to an array, we divide by 16x
number of needed cache lines, because we find 16 values per cache
line. CPU prefetcher can kick nicely.

Also move @cluster_masks to READ_MOSTLY section to avoid false sharing.

Tested on a dual socket host with 256 cpus, cost for a full broadcast
is now 11 usec instead of 33 usec.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211007143556.574911-1-eric.dumazet@gmail.com
This commit is contained in:
Eric Dumazet 2021-10-07 07:35:56 -07:00 committed by Peter Zijlstra
parent 3906fe9bb7
commit cc95a07fef
1 changed files with 21 additions and 6 deletions
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27
arch/x86/kernel/apic/x2apic_cluster.c
View file

@ -15,9 +15,15 @@ struct cluster_mask {
struct cpumask mask;
};
static DEFINE_PER_CPU(u32, x86_cpu_to_logical_apicid);
/*
* __x2apic_send_IPI_mask() possibly needs to read
* x86_cpu_to_logical_apicid for all online cpus in a sequential way.
* Using per cpu variable would cost one cache line per cpu.
*/
static u32 *x86_cpu_to_logical_apicid __read_mostly;
static DEFINE_PER_CPU(cpumask_var_t, ipi_mask);
static DEFINE_PER_CPU(struct cluster_mask *, cluster_masks);
static DEFINE_PER_CPU_READ_MOSTLY(struct cluster_mask *, cluster_masks);
static struct cluster_mask *cluster_hotplug_mask;
static int x2apic_acpi_madt_oem_check(char *oem_id, char *oem_table_id)
@ -27,7 +33,7 @@ static int x2apic_acpi_madt_oem_check(char *oem_id, char *oem_table_id)
static void x2apic_send_IPI(int cpu, int vector)
{
u32 dest = per_cpu(x86_cpu_to_logical_apicid, cpu);
u32 dest = x86_cpu_to_logical_apicid[cpu];
/* x2apic MSRs are special and need a special fence: */
weak_wrmsr_fence();
@ -58,7 +64,7 @@ __x2apic_send_IPI_mask(const struct cpumask *mask, int vector, int apic_dest)
dest = 0;
for_each_cpu_and(clustercpu, tmpmsk, &cmsk->mask)
dest |= per_cpu(x86_cpu_to_logical_apicid, clustercpu);
dest |= x86_cpu_to_logical_apicid[clustercpu];
if (!dest)
continue;
@ -94,7 +100,7 @@ static void x2apic_send_IPI_all(int vector)
static u32 x2apic_calc_apicid(unsigned int cpu)
{
return per_cpu(x86_cpu_to_logical_apicid, cpu);
return x86_cpu_to_logical_apicid[cpu];
}
static void init_x2apic_ldr(void)
@ -103,7 +109,7 @@ static void init_x2apic_ldr(void)
u32 cluster, apicid = apic_read(APIC_LDR);
unsigned int cpu;
this_cpu_write(x86_cpu_to_logical_apicid, apicid);
x86_cpu_to_logical_apicid[smp_processor_id()] = apicid;
if (cmsk)
goto update;
@ -166,12 +172,21 @@ static int x2apic_dead_cpu(unsigned int dead_cpu)
static int x2apic_cluster_probe(void)
{
u32 slots;
if (!x2apic_mode)
return 0;
slots = max_t(u32, L1_CACHE_BYTES/sizeof(u32), nr_cpu_ids);
x86_cpu_to_logical_apicid = kcalloc(slots, sizeof(u32), GFP_KERNEL);
if (!x86_cpu_to_logical_apicid)
return 0;
if (cpuhp_setup_state(CPUHP_X2APIC_PREPARE, "x86/x2apic:prepare",
x2apic_prepare_cpu, x2apic_dead_cpu) < 0) {
pr_err("Failed to register X2APIC_PREPARE\n");
kfree(x86_cpu_to_logical_apicid);
x86_cpu_to_logical_apicid = NULL;
return 0;
}
init_x2apic_ldr();