On Thu, Dec 29, 2011 at 2:39 PM, Marek Szyprowski m.szyprowski@samsung.com wrote:
From: Michal Nazarewicz mina86@mina86.com
When set_migratetype_isolate() sets pageblock's migrate type, it does not change each page_private data. This makes sense, as the function has no way of knowing what kind of information page_private stores.
A side effect is that instead of draining pages from all zones, set_migratetype_isolate() now drain only pages from zone pageblock it operates on is in.
+/* Caller must hold zone->lock. */ +static void __zone_drain_local_pages(void *arg) +{
struct per_cpu_pages *pcp;struct zone *zone = arg;unsigned long flags;local_irq_save(flags);pcp = &per_cpu_ptr(zone->pageset, smp_processor_id())->pcp;if (pcp->count) {/* Caller holds zone->lock, no need to grab it. */__free_pcppages_bulk(zone, pcp->count, pcp);pcp->count = 0;}local_irq_restore(flags);+}
+/*
- Like drain_all_pages() but operates on a single zone. Caller must
- hold zone->lock.
- */
+static void __zone_drain_all_pages(struct zone *zone) +{
on_each_cpu(__zone_drain_local_pages, zone, 1);+}
On Sun, 01 Jan 2012 08:49:13 +0100, Gilad Ben-Yossef gilad@benyossef.com wrote:
Please consider whether sending an IPI to all processors in the system and interrupting them is appropriate here.
You seem to assume that it is probable that each CPU of the possibly 4,096 (MAXSMP on x86) has a per-cpu page for the specified zone,
I'm not really assuming that (in fact I expect what you fear, ie. that most CPUs won't have pages from specified zone an PCP list), however, I really need to make sure to get them off all PCP lists.
otherwise you're just interrupting them out of doing something useful, or save power idle for nothing.
Exactly what's happening now anyway.
While that may or may not be a reasonable assumption for the general drain_all_pages that drains pcps from all zones, I feel it is less likely to be the right thing once you limit the drain to a single zone.
Currently, set_migratetype_isolate() seem to do more then it needs to, ie. it drains all the pages even though all it cares about is a single zone.
Some background on my attempt to reduce "IPI noise" in the system in this context is probably useful here as well: https://lkml.org/lkml/2011/11/22/133
Looks interesting, I'm not entirely sure why it does not end up a race condition, but in case of __zone_drain_all_pages() we already hold zone->lock, so my fears are somehow gone.. I'll give it a try, and prepare a patch for __zone_drain_all_pages().