From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7C962C4727E for ; Wed, 30 Sep 2020 17:04:07 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id CD15E20754 for ; Wed, 30 Sep 2020 17:04:06 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="Q/vzMXcY" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CD15E20754 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=joelfernandes.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 0E9E26B0071; Wed, 30 Sep 2020 13:04:06 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 06EFD8E0001; Wed, 30 Sep 2020 13:04:06 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id EEA5C6B0073; Wed, 30 Sep 2020 13:04:05 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0023.hostedemail.com [216.40.44.23]) by kanga.kvack.org (Postfix) with ESMTP id D5E356B0071 for ; Wed, 30 Sep 2020 13:04:05 -0400 (EDT) Received: from smtpin17.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 8E83218159CED for ; Wed, 30 Sep 2020 17:04:05 +0000 (UTC) X-FDA: 77320350450.17.north75_3f0b53427194 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin17.hostedemail.com (Postfix) with ESMTP id A628618158AEE for ; Wed, 30 Sep 2020 17:03:43 +0000 (UTC) X-HE-Tag: north75_3f0b53427194 X-Filterd-Recvd-Size: 7259 Received: from mail-il1-f193.google.com (mail-il1-f193.google.com [209.85.166.193]) by imf14.hostedemail.com (Postfix) with ESMTP for ; Wed, 30 Sep 2020 17:03:42 +0000 (UTC) Received: by mail-il1-f193.google.com with SMTP id f15so2459713ilj.2 for ; Wed, 30 Sep 2020 10:03:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=/8mrTDb/+4C9kt/OFrFWQ4KoVsWdxaBzMsz+MNCGDv4=; b=Q/vzMXcYGsM37mtX/zsNK3/+zVU6kxcEOtcPXrPi9N5z2tYvX1jvRPm0R1MdSDX4fV /DYpI71tvRg5Qu0u6Tb6MXv3+SaUHPuU29JKJP7S/rp2GXczJV9kJOktRtR+t1T858UV NbXDm/bY0wwGJa7A/CWEzkNDoWvXBPN9xI3q8= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=/8mrTDb/+4C9kt/OFrFWQ4KoVsWdxaBzMsz+MNCGDv4=; b=iL2kdEFiKjZlpbnbbyJxYUHSKSE+4GoAL2Ndqxh/8OFyYEHmWw75DBq1AfsJxg0cas XTaxDMd2tHN93UfoPFPIroAuapYkh03UbAUOGFebcyqvMzX2Au6T8SP6uHv586xN7/GS BClipCmnMgUL+f6UEa55DUmhve6vXwo4hSBqQNDGq7Yfzlptbn0TkB1tOZgZ+mAyz5cQ 9V5WCP+qepEKqR3bhB4iUVa1hQ3n+hDzI2LBFpTwWnooJLAFDRgj2ZLmxqdh6Ypz7XyE otUUCT0D6j7Gk1AOUWrpC47wEfsnDOBIPJa1fVgpBzE2V0ziO351NXRn26tpF5HGz7TE /6zw== X-Gm-Message-State: AOAM530uMyDy9X3hlx8ceoFEY0Ayb/uT+AX4j++3rrAez7gFjRHMP0FA ihhK9aExtpujj85rjJoM2PN3vjzfjJ3zcjY2mDpISw== X-Google-Smtp-Source: ABdhPJxutkr1DdLu49sPIlC6A2hL23UJIbkDO1DoXzlFkslrWFmM/cN5bd/j+OfTumzQJtNkcSCHrsgv+IZpDhzLw28= X-Received: by 2002:a92:d842:: with SMTP id h2mr3125152ilq.176.1601485421339; Wed, 30 Sep 2020 10:03:41 -0700 (PDT) MIME-Version: 1.0 References: <20200922075002.GU12990@dhcp22.suse.cz> <20200922131257.GA29241@pc636> <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> In-Reply-To: <20200930164822.GX2277@dhcp22.suse.cz> From: Joel Fernandes Date: Wed, 30 Sep 2020 13:03:29 -0400 Message-ID: Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func. To: Michal Hocko 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 Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. 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 ? Thanks, - Joel