linux-kernel - Re: [PATCH 2/2] mm: compaction: Minimise the time IRQs are disabled while isolating pages for migration

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Date:	Sat, 26 Feb 2011 01:16:11 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arthur Marsh <arthur.marsh@...ernode.on.net>,
	Clemens Ladisch <cladisch@...glemail.com>,
	Linux-MM <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: compaction: Minimise the time IRQs are
 disabled while isolating pages for migration

On Fri, Feb 25, 2011 at 11:32:04PM +0100, Johannes Weiner wrote:
> I don't understand why this conditional is broken up like this.
> cond_resched() will have the right checks anyway.  Okay, you would
> save fatal_signal_pending() in the 'did one cluster' case.  Is it that
> expensive?  Couldn't this be simpler like
> 
> 		did_cluster = ((low_pfn + 1) % SWAP_CLUSTER_MAX) == 0
> 		lock_contended = spin_is_contended(&zone->lru_lock);
> 		if (did_cluster || lock_contended || need_resched()) {
> 			spin_unlock_irq(&zone->lru_lock);
> 			cond_resched();
> 			spin_lock_irq(&zone->lru_lock);
> 			if (fatal_signal_pending(current))
> 				break;
> 		}
> 
> instead?

If we don't release irqs first, how can need_resched get set in the
first place if the local apic irq can't run? I guess that's why
there's no cond_resched_lock_irq. BTW, I never liked too much clearing
interrupts for locks that can't ever be taken from irq (it's a
scalability boost to reduce contention but it makes things like above
confusing and it increases irq latency a bit.
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