linux-kernel - Re: [PATCH 03/10] mm, compaction: periodically drop lock and restore IRQs in scanners

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Date:	Wed, 11 Jun 2014 13:24:43 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Minchan Kim <minchan@...nel.org>
CC:	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 03/10] mm, compaction: periodically drop lock and restore
 IRQs in scanners

On 06/11/2014 03:32 AM, Minchan Kim wrote:
>> >+	if (cc->mode == MIGRATE_ASYNC) {
>> >+		if (need_resched()) {
>> >+			cc->contended = COMPACT_CONTENDED_SCHED;
>> >+			return true;
>> >  		}
>> >-
>> >+		if (spin_is_locked(lock)) {
> Why do you use spin_is_locked instead of spin_is_contended?

Because I know I have dropped the lock. AFAIK spin_is_locked() means 
somebody else is holding it, which would be a contention for me if I 
would want to take it back. spin_is_contended() means that somebody else 
#1 is holding it AND somebody else #2 is already waiting for it.

Previously in should_release_lock() the code assumed that it was me who 
holds the lock, so I check if somebody else is waiting for it, hence 
spin_is_contended().

But note that the assumption was not always true when 
should_release_lock() was called from compact_checklock_irqsave(). So it 
was another subtle suboptimality. In async compaction when I don't have 
the lock, I should be deciding if I take it based on if somebody else is 
holding it. Instead it was deciding based on if somebody else #1 is 
holding it and somebody else #2 is waiting.
Then there's still a chance of race between this check and call to 
spin_lock_irqsave, so I could spin on the lock even if I don't want to. 
Using spin_trylock_irqsave() instead is like checking spin_is_locked() 
and locking, without this race.

So even though I will probably remove the spin_is_locked() check per 
David's objection, the trylock will still nicely prevent waiting on the 
lock in async compaction.
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