Re: [PATCH 1/2] mm: serialize OOM kill operations

[Dave Peterson <dsp@llnl.gov>]

On Tuesday 25 April 2006 21:10, Nick Piggin wrote:
> Firstly why not use a semaphore and trylocks instead of your homebrew
> lock?

Are you suggesting something like this?

	spinlock_t oom_kill_lock = SPIN_LOCK_UNLOCKED;

	static inline int oom_kill_start(void)
	{
		return !spin_trylock(&oom_kill_lock);
	}

	static inline void oom_kill_finish()
	{
		spin_unlock(&oom_kill_lock);
	}

If you prefer the above implementation, I can rework the patch as
above.

> Second, can you arrange it without using the extra field in mm_struct
> and operation in the mmput fast path?

I'm open to suggestions on other ways of implementing this.  However I
think the performance impact of the proposed implementation should be
miniscule.  The code added to mmput() executes only when the referece
count has reached 0; not on every decrement of the reference count.
Once the reference count has reached 0, the common-case behavior is
still only testing a boolean flag followed by a not-taken branch.  The
use of unlikely() should help the compiler and CPU branch prediction
hardware minimize overhead in the typical case where oom_kill_finish()
is not called.

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