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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Dave Peterson wrote:

>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.
>

I think you need a semaphore? Either way, drop the trivial wrappers.

>
>>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.
>

Mainly the cost of increasing cacheline footprint. I think someone
suggested using a flag bit somewhere... that'd be preferable.

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