From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH 2/4] kernel.h: Add non_block_start/end() Date: Mon, 10 Dec 2018 16:01:59 +0100 Message-ID: <20181210150159.GR1286@dhcp22.suse.cz> References: <20181210103641.31259-1-daniel.vetter@ffwll.ch> <20181210103641.31259-3-daniel.vetter@ffwll.ch> <20181210141337.GQ1286@dhcp22.suse.cz> <20181210144711.GN5289@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20181210144711.GN5289@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: Daniel Vetter , Intel Graphics Development , DRI Development , LKML , linux-mm@kvack.org, Andrew Morton , David Rientjes , Christian =?iso-8859-1?Q?K=F6nig?= , =?iso-8859-1?B?Suly9G1l?= Glisse , Daniel Vetter List-Id: intel-gfx@lists.freedesktop.org On Mon 10-12-18 15:47:11, Peter Zijlstra wrote: > On Mon, Dec 10, 2018 at 03:13:37PM +0100, Michal Hocko wrote: > > I do not see any scheduler guys Cced and it would be really great to get > > their opinion here. > > > > On Mon 10-12-18 11:36:39, Daniel Vetter wrote: > > > In some special cases we must not block, but there's not a > > > spinlock, preempt-off, irqs-off or similar critical section already > > > that arms the might_sleep() debug checks. Add a non_block_start/end() > > > pair to annotate these. > > > > > > This will be used in the oom paths of mmu-notifiers, where blocking is > > > not allowed to make sure there's forward progress. > > > > Considering the only alternative would be to abuse > > preempt_{disable,enable}, and that really has a different semantic, I > > think this makes some sense. The cotext is preemptible but we do not > > want notifier to sleep on any locks, WQ etc. > > I'm confused... what is this supposed to do? > > And what does 'block' mean here? Without preempt_disable/IRQ-off we're > subject to regular preemption and execution can stall for arbitrary > amounts of time. The notifier is called from quite a restricted context - oom_reaper - which shouldn't depend on any locks or sleepable conditionals. The code should be swift as well but we mostly do care about it to make a forward progress. Checking for sleepable context is the best thing we could come up with that would describe these demands at least partially. -- Michal Hocko SUSE Labs