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

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Date:	Tue, 1 Mar 2011 08:25:08 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
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 Tue, Mar 01, 2011 at 12:07:12AM +0100, Andrea Arcangeli wrote:
> On Tue, Mar 01, 2011 at 08:01:31AM +0900, Minchan Kim wrote:
> > I am not sure it's good if we release the lock whenever lru->lock was contended
> > unconditionally? There are many kinds of lru_lock operations(add to lru, 
> > del from lru, isolation, reclaim, activation, deactivation and so on).
> 
> This is mostly to mirror cond_resched_lock (which actually uses
> spin_needbreak but it's ok to have it also when preempt is off). I
> doubt it makes a big difference but I tried to mirror
> cond_resched_lock.

But what's the benefit of releasing lock in here when lock contentionn happen where
activate_page for example? 
> 
> > Do we really need to release the lock whenever all such operations were contened?
> > I think what we need is just spin_is_contended_irqcontext.
> > Otherwise, please write down the comment for justifying for it.
> 
> What is spin_is_contended_irqcontext?

I thought what we need function is to check lock contention happened in only irq context
for short irq latency.

> 
> > This patch is for reducing for irq latency but do we have to check signal 
> > in irq hold time?
> 
> I think it's good idea to check the signal in case the loop is very
> long and this is run in direct compaction context.

I don't oppose the signal check.
I am not sure why we should check by just sign of lru_lock contention.

How about this by coarse-grained?

               /* give a chance to irqs before checking need_resched() */
               if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
                       if (fatal_signal_pending(current))
                               break;
                       spin_unlock_irq(&zone->lru_lock);
                       unlocked = true;
               }
               if (need_resched() || spin_is_contended(&zone->lru_lock)) {
                       if (!unlocked)
                               spin_unlock_irq(&zone->lru_lock);
                       cond_resched();
                       spin_lock_irq(&zone->lru_lock);
               } else if (unlocked)
                       spin_lock_irq(&zone->lru_lock);


> 
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-- 
Kind regards,
Minchan Kim
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