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

[Nick Piggin]

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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