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Date: Sun, 01 Jan 2012 16:54:38 +0100 From: "Michal Nazarewicz" <mina86@...a86.com> To: "Marek Szyprowski" <m.szyprowski@...sung.com>, "Gilad Ben-Yossef" <gilad@...yossef.com> Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org, linux-mm@...ck.org, linaro-mm-sig@...ts.linaro.org, "Kyungmin Park" <kyungmin.park@...sung.com>, "Russell King" <linux@....linux.org.uk>, "Andrew Morton" <akpm@...ux-foundation.org>, "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>, "Daniel Walker" <dwalker@...eaurora.org>, "Mel Gorman" <mel@....ul.ie>, "Arnd Bergmann" <arnd@...db.de>, "Jesse Barker" <jesse.barker@...aro.org>, "Jonathan Corbet" <corbet@....net>, "Shariq Hasnain" <shariq.hasnain@...aro.org>, "Chunsang Jeong" <chunsang.jeong@...aro.org>, "Dave Hansen" <dave@...ux.vnet.ibm.com>, "Benjamin Gaignard" <benjamin.gaignard@...aro.org> Subject: Re: [PATCH 01/11] mm: page_alloc: set_migratetype_isolate: drain PCP prior to isolating > On Thu, Dec 29, 2011 at 2:39 PM, Marek Szyprowski > <m.szyprowski@...sung.com> wrote: >> From: Michal Nazarewicz <mina86@...a86.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@...yossef.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(). -- Best regards, _ _ .o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o ..o | Computer Science, Michał “mina86” Nazarewicz (o o) ooo +----<email/xmpp: mpn@...gle.com>--------------ooO--(_)--Ooo-- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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