On Mon, 27 Mar 2017 10:55:14 +0200
Jesper Dangaard Brouer <brouer@xxxxxxxxxx> wrote:
> A possible solution, would be use the local_bh_{disable,enable} instead
> of the {preempt_disable,enable} calls. But it is slower, using numbers
> from [1] (19 vs 11 cycles), thus the expected cycles saving is 38-19=19.
>
> The problematic part of using local_bh_enable is that this adds a
> softirq/bottom-halves rescheduling point (as it checks for pending
> BHs). Thus, this might affects real workloads.
I implemented this solution in patch below... and tested it on mlx5 at
50G with manually disabled driver-page-recycling. It works for me.
To Mel, that do you prefer... a partial-revert or something like this?
[PATCH] mm, page_alloc: re-enable softirq use of per-cpu page allocator
From: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
IRQ context were excluded from using the Per-Cpu-Pages (PCP) lists
caching of order-0 pages in commit 374ad05ab64d ("mm, page_alloc: only
use per-cpu allocator for irq-safe requests").
This unfortunately also included excluded SoftIRQ. This hurt the
performance for the use-case of refilling DMA RX rings in softirq
context.
This patch re-allow softirq context, which should be safe by disabling
BH/softirq, while accessing the list. And makes sure to avoid
PCP-lists access from both hard-IRQ and NMI context.
One concern with this change is adding a BH (enable) scheduling point
at both PCP alloc and free.
Fixes: 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe requests")
---
include/trace/events/kmem.h | 2 ++
mm/page_alloc.c | 41 ++++++++++++++++++++++++++++++++++-------
2 files changed, 36 insertions(+), 7 deletions(-)
diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index 6b2e154fd23a..ad412ad1b092 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -244,6 +244,8 @@ DECLARE_EVENT_CLASS(mm_page,
__entry->order,
__entry->migratetype,
__entry->order == 0)
+// WARNING: percpu_refill check not 100% correct after commit
+// 374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe requests")
);
DEFINE_EVENT(mm_page, mm_page_alloc_zone_locked,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6cbde310abed..db9ffc8ac538 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2470,6 +2470,25 @@ void mark_free_pages(struct zone *zone)
}
#endif /* CONFIG_PM */
+static __always_inline int in_irq_or_nmi(void)
+{
+ return in_irq() || in_nmi();
+// XXX: hoping compiler will optimize this (todo verify) into:
+// #define in_irq_or_nmi() (preempt_count() & (HARDIRQ_MASK | NMI_MASK))
+
+ /* compiler was smart enough to only read __preempt_count once
+ * but added two branches
+asm code:
+ │ mov __preempt_count,%eax
+ │ test $0xf0000,%eax // HARDIRQ_MASK: 0x000f0000
+ │ ┌──jne 2a
+ │ │ test $0x100000,%eax // NMI_MASK: 0x00100000
+ │ │↓ je 3f
+ │ 2a:└─→mov %rbx,%rdi
+
+ */
+}
+
/*
* Free a 0-order page
* cold == true ? free a cold page : free a hot page
@@ -2481,7 +2500,11 @@ void free_hot_cold_page(struct page *page, bool cold)
unsigned long pfn = page_to_pfn(page);
int migratetype;
- if (in_interrupt()) {
+ /*
+ * Exclude (hard) IRQ and NMI context from using the pcplists.
+ * But allow softirq context, via disabling BH.
+ */
+ if (in_irq_or_nmi()) {
__free_pages_ok(page, 0);
return;
}
@@ -2491,7 +2514,7 @@ void free_hot_cold_page(struct page *page, bool cold)
migratetype = get_pfnblock_migratetype(page, pfn);
set_pcppage_migratetype(page, migratetype);
- preempt_disable();
+ local_bh_disable();
/*
* We only track unmovable, reclaimable and movable on pcp lists.
@@ -2522,7 +2545,7 @@ void free_hot_cold_page(struct page *page, bool cold)
}
out:
- preempt_enable();
+ local_bh_enable();
}
/*
@@ -2647,7 +2670,7 @@ static struct page *__rmqueue_pcplist(struct zone *zone, int migratetype,
{
struct page *page;
- VM_BUG_ON(in_interrupt());
+ VM_BUG_ON(in_irq());
do {
if (list_empty(list)) {
@@ -2680,7 +2703,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
bool cold = ((gfp_flags & __GFP_COLD) != 0);
struct page *page;
- preempt_disable();
+ local_bh_disable();
pcp = &this_cpu_ptr(zone->pageset)->pcp;
list = &pcp->lists[migratetype];
page = __rmqueue_pcplist(zone, migratetype, cold, pcp, list);
@@ -2688,7 +2711,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
__count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
zone_statistics(preferred_zone, zone);
}
- preempt_enable();
+ local_bh_enable();
return page;
}
@@ -2704,7 +2727,11 @@ struct page *rmqueue(struct zone *preferred_zone,
unsigned long flags;
struct page *page;
- if (likely(order == 0) && !in_interrupt()) {
+ /*
+ * Exclude (hard) IRQ and NMI context from using the pcplists.
+ * But allow softirq context, via disabling BH.
+ */
+ if (likely(order == 0) && !in_irq_or_nmi() ) {
page = rmqueue_pcplist(preferred_zone, zone, order,
gfp_flags, migratetype);
goto out;
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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