From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Peterson Subject: Re: [PATCH 1/2] mm: serialize OOM kill operations Date: Thu, 27 Apr 2006 09:56:15 -0700 References: <200604251701.31899.dsp@llnl.gov> <200604261014.15008.dsp@llnl.gov> <44503BA2.7000405@yahoo.com.au> In-Reply-To: <44503BA2.7000405@yahoo.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200604270956.15658.dsp@llnl.gov> Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, riel@surriel.com, akpm@osdl.org List-ID: 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... Dave -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org