Despite 1db7829e5657 ("x86/hpet: do local APIC EOI after interrupt
processing") we can still observe nested invocations of
hpet_interrupt_handler(). This is, afaict, a result of previously used
channels retaining their IRQ affinity until some other CPU re-uses them.
Such nesting is increasingly problematic with higher CPU counts, as both
handle_hpet_broadcast() and cpumask_raise_softirq() have a cpumask_t local
variable. IOW already a single level of nesting may require more stack
space (2 times above 4k) than we have available (8k), when NR_CPUS=16383
(the maximum value presently possible).Signed-off-by: Jan Beulich <[email protected]> --- Whether this is still worthwhile with "x86/HPET: use single, global, low- priority vector for broadcast IRQ" isn't quite clear to me. --- a/xen/arch/x86/hpet.c +++ b/xen/arch/x86/hpet.c @@ -442,6 +442,8 @@ static void __init hpet_fsb_cap_lookup(v num_hpets_used, num_chs); } +static DEFINE_PER_CPU(struct hpet_event_channel *, lru_channel); + static struct hpet_event_channel *hpet_get_channel(unsigned int cpu) { static unsigned int next_channel; @@ -454,9 +456,21 @@ static struct hpet_event_channel *hpet_g if ( num_hpets_used >= nr_cpu_ids ) return &hpet_events[cpu]; + /* + * Try the least recently used channel first. It may still have its IRQ's + * affinity set to the desired CPU. This way we also limit having multiple + * of our IRQs raised on the same CPU, in possibly a nested manner. + */ + ch = per_cpu(lru_channel, cpu); + if ( ch && !test_and_set_bit(HPET_EVT_USED_BIT, &ch->flags) ) + { + ch->cpu = cpu; + return ch; + } + + /* Then look for an unused channel. */ next = arch_fetch_and_add(&next_channel, 1) % num_hpets_used; - /* try unused channel first */ for ( i = next; i < next + num_hpets_used; i++ ) { ch = &hpet_events[i % num_hpets_used]; @@ -479,6 +493,8 @@ static void set_channel_irq_affinity(str { struct irq_desc *desc = irq_to_desc(ch->msi.irq); + per_cpu(lru_channel, ch->cpu) = ch; + ASSERT(!local_irq_is_enabled()); spin_lock(&desc->lock); hpet_msi_mask(desc);
