Messages in this thread | | | From | Dave Peterson <> | Subject | Re: [PATCH 1/2] mm: serialize OOM kill operations | Date | Thu, 27 Apr 2006 09:56:15 -0700 |
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On Wednesday 26 April 2006 20:33, Nick Piggin wrote: > Dave Peterson wrote: > >If you prefer the above implementation, I can rework the patch as > >above. > > I think you need a semaphore?
In this particular case, I think a semaphore is unnecessary because we just want out_of_memory() to return to its caller if an OOM kill is already in progress (as opposed to waiting in out_of_memory() and then starting a new OOM kill operation). What I want to avoid is the the following type of behavior:
1. Two processes (A and B) call out_of_memory() at roughly the same time and race for oom_kill_lock. Let's say A wins and B is delayed.
2. Process A shoots some process and releases oom_kill_lock.
3. Process B now acquires oom_kill_lock and shoots another process. However this isn't really what we want to do if the OOM kill done by A above freed enough memory to resolve the OOM condition.
> Either way, drop the trivial wrappers.
Ok, I'll drop the 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.
Ok, I'll add a ->flags member to mm_struct and just use one bit for the oom_notify value. Then if other users of mm_struct need flag bits for other things in the future they can all share ->flags. I'll rework my patches and repost shortly...
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