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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 203ECC433DF for ; Thu, 28 May 2020 18:34:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EE321206A1 for ; Thu, 28 May 2020 18:34:35 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Q1fosZG8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405830AbgE1Sef (ORCPT ); Thu, 28 May 2020 14:34:35 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:47150 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2405775AbgE1Sed (ORCPT ); Thu, 28 May 2020 14:34:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1590690871; 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=YNkxNQgU0Bvr5BvVxj5dKt6vrXoZ9dGRx/rhFTl7OMQ=; b=Q1fosZG8a+kGRfpNConCIzwkTJjlze9Ik/CkeLfNkRSjTOQJvMHovZIORXrDdLh85YA0rO BkaSKag9x0LGjQFRcm8j2GSwAd+VgP7zyjoyqzdXAqP2qcyDC+NrlKpL/i260vYEsH6lN/ 2kWST9eiX8Jo1wRrUYDmneD9k/gkfrM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-93-762b9mjwMgekqrZ8HlQw0g-1; Thu, 28 May 2020 14:34:28 -0400 X-MC-Unique: 762b9mjwMgekqrZ8HlQw0g-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 84E3F1005510; Thu, 28 May 2020 18:34:25 +0000 (UTC) Received: from lorien.usersys.redhat.com (ovpn-114-31.phx2.redhat.com [10.3.114.31]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 907ECA1031; Thu, 28 May 2020 18:34:23 +0000 (UTC) Date: Thu, 28 May 2020 14:34:21 -0400 From: Phil Auld To: Peter Zijlstra Cc: Joel Fernandes , Nishanth Aravamudan , Julien Desfossez , Tim Chen , mingo@kernel.org, tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org, vpillai , linux-kernel@vger.kernel.org, fweisbec@gmail.com, keescook@chromium.org, Aaron Lu , Aubrey Li , aubrey.li@linux.intel.com, Valentin Schneider , Mel Gorman , Pawan Gupta , Paolo Bonzini Subject: Re: [PATCH RFC] sched: Add a per-thread core scheduling interface Message-ID: <20200528183421.GD26442@lorien.usersys.redhat.com> References: <20200520222642.70679-1-joel@joelfernandes.org> <20200521085122.GF325280@hirez.programming.kicks-ass.net> <20200521134705.GA140701@google.com> <20200522125905.GM325280@hirez.programming.kicks-ass.net> <20200522213524.GD213825@google.com> <20200524140046.GA5598@lorien.usersys.redhat.com> <20200528170128.GN2483@worktop.programming.kicks-ass.net> <20200528181715.GC26442@lorien.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200528181715.GC26442@lorien.usersys.redhat.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 28, 2020 at 02:17:19PM -0400 Phil Auld wrote: > On Thu, May 28, 2020 at 07:01:28PM +0200 Peter Zijlstra wrote: > > On Sun, May 24, 2020 at 10:00:46AM -0400, Phil Auld wrote: > > > On Fri, May 22, 2020 at 05:35:24PM -0400 Joel Fernandes wrote: > > > > On Fri, May 22, 2020 at 02:59:05PM +0200, Peter Zijlstra wrote: > > > > [..] > > > > > > > It doens't allow tasks for form their own groups (by for example setting > > > > > > > the key to that of another task). > > > > > > > > > > > > So for this, I was thinking of making the prctl pass in an integer. And 0 > > > > > > would mean untagged. Does that sound good to you? > > > > > > > > > > A TID, I think. If you pass your own TID, you tag yourself as > > > > > not-sharing. If you tag yourself with another tasks's TID, you can do > > > > > ptrace tests to see if you're allowed to observe their junk. > > > > > > > > But that would require a bunch of tasks agreeing on which TID to tag with. > > > > For example, if 2 tasks tag with each other's TID, then they would have > > > > different tags and not share. > > > > Well, don't do that then ;-) > > > > That was a poorly worded example :) > Heh, sorry, I thought that was my statement. I do not mean to belittle Joel's example... That's a fine example of a totally different problem than I was thinking of :) Cheers, Phil > The point I was trying to make was more that one TID of a group (not cgroup!) > of tasks is just an arbitrary value. > > At a single process (or pair rather) level, sure, you can see it as an > identifier of whom you want to share with, but even then you have to tag > both processes with this. And it has less meaning when the whom you want to > share with is mutltiple tasks. > > > > > What's wrong with passing in an integer instead? In any case, we would do the > > > > CAP_SYS_ADMIN check to limit who can do it. > > > > So the actual permission model can be different depending on how broken > > the hardware is. > > > > > > Also, one thing CGroup interface allows is an external process to set the > > > > cookie, so I am wondering if we should use sched_setattr(2) instead of, or in > > > > addition to, the prctl(2). That way, we can drop the CGroup interface > > > > completely. How do you feel about that? > > > > > > > > > > I think it should be an arbitrary 64bit value, in both interfaces to avoid > > > any potential reuse security issues. > > > > > > I think the cgroup interface could be extended not to be a boolean but take > > > the value. With 0 being untagged as now. > > > > How do you avoid reuse in such a huge space? That just creates yet > > another problem for the kernel to keep track of who is who. > > > > The kernel doesn't care or have to track anything. The admin does. > At the kernel level it's just matching cookies. > > Tasks A,B,C all can share core so you give them each A's TID as a cookie. > Task A then exits. Now B and C are using essentially a random value. > Task D comes along and want to share with B and C. You have to tag it > with A's old TID, which has no meaning at this point. > > And if A's TID ever gets reused. The new A` gets to share too. At some > level aren't those still 32bits? > > > With random u64 numbers, it even becomes hard to determine if you're > > sharing at all or not. > > > > Now, with the current SMT+MDS trainwreck, any sharing is bad because it > > allows leaking kernel privates. But under a less severe thread scenario, > > say where only user data would be at risk, the ptrace() tests make > > sense, but those become really hard with random u64 numbers too. > > > > What would the purpose of random u64 values be for cgroups? That only > > replicates the problem of determining uniqueness there. Then you can get > > two cgroups unintentionally sharing because you got lucky. > > > > Seems that would be more flexible for the admin. > > What if you had two cgroups you wanted to allow to run together? Or a > cgroup and a few processes from a different one (say with different > quotas or something). > > I don't have such use cases so I don't feel that strongly but it seemed > more flexible and followed the mechanism-in-kernel/policy-in-userspace > dictum rather than basing the functionality on the implementation details. > > > Cheers, > Phil > > > > Also, fundamentally, we cannot have more threads than TID space, it's a > > natural identifier. > > > > -- --