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[23.128.96.18]) by mx.google.com with ESMTP id s19si1516894edx.480.2020.09.30.10.23.27; Wed, 30 Sep 2020 10:23:49 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@suse.com header.s=susede1 header.b=FSDwYdCZ; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=suse.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731335AbgI3RWI (ORCPT + 99 others); Wed, 30 Sep 2020 13:22:08 -0400 Received: from mx2.suse.de ([195.135.220.15]:42558 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726540AbgI3RWI (ORCPT ); Wed, 30 Sep 2020 13:22:08 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1601486526; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=4JoyM1WcMGgZXIw9gw4Exp5gsg0i/SR2yP+vZX8kElM=; b=FSDwYdCZbuyQbWwctDAQH5cDdse6mMuBHf1qQIVi76Tay1fWLHO2hCgx2TYq6S/BIIwmbw DD3NPUO6/BDWK58fJSFDHXBwWZkTaMJpk1YzcjRNxNvXj6UtOF1XbAufCcY03UHv5SjOEu eZ659X4lrCSdYUZz+PugtmqNPbo5Tqw= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 1ABB5ABE3; Wed, 30 Sep 2020 17:22:06 +0000 (UTC) Date: Wed, 30 Sep 2020 19:22:05 +0200 From: Michal Hocko To: Joel Fernandes Cc: Uladzislau Rezki , Mel Gorman , "Paul E. McKenney" , LKML , RCU , linux-mm , Andrew Morton , Peter Zijlstra , Vlastimil Babka , Thomas Gleixner , "Theodore Y . Ts'o" , Sebastian Andrzej Siewior , Oleksiy Avramchenko , Mel Gorman Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. Message-ID: <20200930172205.GY2277@dhcp22.suse.cz> References: <20200923103706.GJ3179@techsingularity.net> <20200923154105.GO29330@paulmck-ThinkPad-P72> <20200923232251.GK3179@techsingularity.net> <20200924081614.GA14819@pc636> <20200925080503.GC3389@dhcp22.suse.cz> <20200925153129.GB25350@pc636> <20200925154741.GI3389@dhcp22.suse.cz> <20200930152517.GA1470428@google.com> <20200930164822.GX2277@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 30-09-20 13:03:29, Joel Fernandes wrote: > On Wed, Sep 30, 2020 at 12:48 PM Michal Hocko wrote: > > > > On Wed 30-09-20 11:25:17, Joel Fernandes wrote: > > > On Fri, Sep 25, 2020 at 05:47:41PM +0200, Michal Hocko wrote: > > > > On Fri 25-09-20 17:31:29, Uladzislau Rezki wrote: > > > > > > > > > > > > > > > > > > All good points! > > > > > > > > > > > > > > > > > > On the other hand, duplicating a portion of the allocator functionality > > > > > > > > > within RCU increases the amount of reserved memory, and needlessly most > > > > > > > > > of the time. > > > > > > > > > > > > > > > > > > > > > > > > > But it's very similar to what mempools are for. > > > > > > > > > > > > > > > As for dynamic caching or mempools. It requires extra logic on top of RCU > > > > > > > to move things forward and it might be not efficient way. As a side > > > > > > > effect, maintaining of the bulk arrays in the separate worker thread > > > > > > > will introduce other drawbacks: > > > > > > > > > > > > This is true but it is also true that it is RCU to require this special > > > > > > logic and we can expect that we might need to fine tune this logic > > > > > > depending on the RCU usage. We definitely do not want to tune the > > > > > > generic page allocator for a very specific usecase, do we? > > > > > > > > > > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability > > > > > to provide a memory service for contexts which are not allowed to > > > > > sleep, RCU is part of them. Both flags used to provide such ability > > > > > before but not anymore. > > > > > > > > > > Do you agree with it? > > > > > > > > Yes this sucks. But this is something that we likely really want to live > > > > with. We have to explicitly _document_ that really atomic contexts in RT > > > > cannot use the allocator. From the past discussions we've had this is > > > > likely the most reasonable way forward because we do not really want to > > > > encourage anybody to do something like that and there should be ways > > > > around that. The same is btw. true also for !RT. The allocator is not > > > > NMI safe and while we should be able to make it compatible I am not > > > > convinced we really want to. > > > > > > > > Would something like this be helpful wrt documentation? > > > > > > > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h > > > > index 67a0774e080b..9fcd47606493 100644 > > > > --- a/include/linux/gfp.h > > > > +++ b/include/linux/gfp.h > > > > @@ -238,7 +238,9 @@ struct vm_area_struct; > > > > * %__GFP_FOO flags as necessary. > > > > * > > > > * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > > > > - * watermark is applied to allow access to "atomic reserves" > > > > + * watermark is applied to allow access to "atomic reserves". > > > > + * The current implementation doesn't support NMI and other non-preemptive context > > > > + * (e.g. raw_spin_lock). > > > > > > I think documenting is useful. > > > > > > Could it be more explicit in what the issue is? Something like: > > > > > > * Even with GFP_ATOMIC, calls to the allocator can sleep on PREEMPT_RT > > > systems. Therefore, the current low-level allocator implementation does not > > > support being called from special contexts that are atomic on RT - such as > > > NMI and raw_spin_lock. Due to these constraints and considering calling code > > > usually has no control over the PREEMPT_RT configuration, callers of the > > > allocator should avoid calling the allocator from these cotnexts even in > > > non-RT systems. > > > > I do not mind documenting RT specific behavior but as mentioned in other > > reply, this should likely go via RT tree for now. There is likely more > > to clarify about atomicity for PREEMPT_RT. > > I am sorry, I did not understand what you meant by something missing > in Linus Tree. CONFIG_PREEMPT_RT is there. OK, I was not aware we already CONFIG_PREEMPT_RT in the three. There is still a lot from the RT patchset including sleeping spin locks that make a real difference. Or maybe I haven't checked properly. > Could you clarify? Also the CONFIG_PREEMPT_RT is the only thing > driving this requirement for GFP_ATOMIC right? Or are there non-RT > reasons why GFP_ATOMIC allocation cannot be done from > NMI/raw_spin_lock ? I have already sent a clarification patch [1]. NMI is not supported regardless of the preemption mode. Hope this clarifies it a bit. [1] http://lkml.kernel.org/r/20200929123010.5137-1-mhocko@kernel.org -- Michal Hocko SUSE Labs