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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