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McKenney" To: Joel Fernandes Cc: Andy Lutomirski , LKML , Frederic Weisbecker , Jonathan Corbet , Josh Triplett , Android Kernel Team , Lai Jiangshan , "open list:DOCUMENTATION" , Mathieu Desnoyers , Mauro Carvalho Chehab , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC v1 2/2] rcu/tree: Remove dynticks_nmi_nesting counter Message-ID: <20190830004818.GX4125@linux.ibm.com> Reply-To: paulmck@kernel.org References: <20190828220108.GC26530@linux.ibm.com> <20190828221444.GA100789@google.com> <20190828231247.GE26530@linux.ibm.com> <20190829015155.GB100789@google.com> <20190829034336.GD4125@linux.ibm.com> <20190829144355.GE63638@google.com> <20190829160946.GP4125@linux.ibm.com> <20190829165407.GT4125@linux.ibm.com> <20190829190046.GB115245@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190829190046.GB115245@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-30_01:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1034 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=965 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908300009 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 29, 2019 at 03:00:46PM -0400, Joel Fernandes wrote: > Hi Paul, > > On Thu, Aug 29, 2019 at 09:54:07AM -0700, Paul E. McKenney wrote: > > On Thu, Aug 29, 2019 at 09:21:46AM -0700, Andy Lutomirski wrote: > > > On Thu, Aug 29, 2019 at 9:10 AM Paul E. McKenney wrote: > > > > > > > > On Thu, Aug 29, 2019 at 10:43:55AM -0400, Joel Fernandes wrote: > > > > > > > > [ . . . ] > > > > > > > > > Paul, do we also nuke rcu_eqs_special_set()? Currently I don't see anyone > > > > > using it. And also remove the bottom most bit of dynticks? > > > > > > > > > > Also what happens if a TLB flush broadcast is needed? Do we IPI nohz or idle > > > > > CPUs are the moment? > > > > > > > > > > All of this was introduced in: > > > > > b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter") > > > > > > > > Adding Andy Lutomirski on CC. > > > > > > > > Andy, is this going to be used in the near term, or should we just get > > > > rid of it? > > > > > > Let's get rid of it. I'm not actually convinced it *can* be used as designed. > > > > > > For those who forgot the history or weren't cc'd on all of it: I had > > > this clever idea about how we could reduce TLB flushes. I implemented > > > some of it (but not the part that would have used this RCU feature), > > > and it exploded in nasty and subtle ways. This caused me to learn > > > that speculative TLB fills were a problem that I had entirely failed > > > to account for. Then PTI happened and thoroughly muddied the water. > > > > Yeah, PTI was quite annoying. Still is, from what I can see. :-/ > > > > > So I think we should just drop this :( > > > > OK, thank you! I will put a tag into -rcu marking its removal in case > > it should prove useful whenever for whatever. > > > > Joel, would you like to remove this, or would you rather that I did? > > It is in code you are working with right now, so if I do it, I need to > > wait until yours is finalized. Which wouldn't be a problem. > > I can remove it in my series, made a note to do so. Sounds good! Thanx, Paul