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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sat, 10 Aug 2019 04:38:58 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x7A3cv2f8192338 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sat, 10 Aug 2019 03:38:57 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D8A07B2064; Sat, 10 Aug 2019 03:38:57 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A3529B205F; Sat, 10 Aug 2019 03:38:57 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.138.198]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sat, 10 Aug 2019 03:38:57 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id B4C1C16C9A73; Fri, 9 Aug 2019 20:38:57 -0700 (PDT) Date: Fri, 9 Aug 2019 20:38:57 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Byungchul Park , linux-kernel@vger.kernel.org, Rao Shoaib , max.byungchul.park@gmail.com, Davidlohr Bueso , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching Reply-To: paulmck@linux.ibm.com References: <20190807175215.GE28441@linux.ibm.com> <20190808095232.GA30401@X58A-UD3R> <20190808125607.GB261256@google.com> <20190808233014.GA184373@google.com> <20190809151619.GD28441@linux.ibm.com> <20190809153924.GB211412@google.com> <20190809163346.GF28441@linux.ibm.com> <20190809202226.GC255533@google.com> <20190809202645.GD255533@google.com> <20190809212512.GF255533@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190809212512.GF255533@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19081003-0064-0000-0000-000004073C35 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011577; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000287; SDB=6.01244611; UDB=6.00656652; IPR=6.01026095; MB=3.00028115; MTD=3.00000008; XFM=3.00000015; UTC=2019-08-10 03:39:02 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19081003-0065-0000-0000-00003E9CF9E0 Message-Id: <20190810033857.GQ28441@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-10_01:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908100038 Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Fri, Aug 09, 2019 at 05:25:12PM -0400, Joel Fernandes wrote: > On Fri, Aug 09, 2019 at 04:26:45PM -0400, Joel Fernandes wrote: > > On Fri, Aug 09, 2019 at 04:22:26PM -0400, Joel Fernandes wrote: > > > On Fri, Aug 09, 2019 at 09:33:46AM -0700, Paul E. McKenney wrote: > > > > On Fri, Aug 09, 2019 at 11:39:24AM -0400, Joel Fernandes wrote: > > > > > On Fri, Aug 09, 2019 at 08:16:19AM -0700, Paul E. McKenney wrote: > > > > > > On Thu, Aug 08, 2019 at 07:30:14PM -0400, Joel Fernandes wrote: > > > > > [snip] > > > > > > > > But I could make it something like: > > > > > > > > 1. Letting ->head grow if ->head_free busy > > > > > > > > 2. If head_free is busy, then just queue/requeue the monitor to try again. > > > > > > > > > > > > > > > > This would even improve performance, but will still risk going out of memory. > > > > > > > > > > > > > > It seems I can indeed hit an out of memory condition once I changed it to > > > > > > > "letting list grow" (diff is below which applies on top of this patch) while > > > > > > > at the same time removing the schedule_timeout(2) and replacing it with > > > > > > > cond_resched() in the rcuperf test. I think the reason is the rcuperf test > > > > > > > starves the worker threads that are executing in workqueue context after a > > > > > > > grace period and those are unable to get enough CPU time to kfree things fast > > > > > > > enough. But I am not fully sure about it and need to test/trace more to > > > > > > > figure out why this is happening. > > > > > > > > > > > > > > If I add back the schedule_uninterruptibe_timeout(2) call, the out of memory > > > > > > > situation goes away. > > > > > > > > > > > > > > Clearly we need to do more work on this patch. > > > > > > > > > > > > > > In the regular kfree_rcu_no_batch() case, I don't hit this issue. I believe > > > > > > > that since the kfree is happening in softirq context in the _no_batch() case, > > > > > > > it fares better. The question then I guess is how do we run the rcu_work in a > > > > > > > higher priority context so it is not starved and runs often enough. I'll > > > > > > > trace more. > > > > > > > > > > > > > > Perhaps I can also lower the priority of the rcuperf threads to give the > > > > > > > worker thread some more room to run and see if anything changes. But I am not > > > > > > > sure then if we're preparing the code for the real world with such > > > > > > > modifications. > > > > > > > > > > > > > > Any thoughts? > > > > > > > > > > > > Several! With luck, perhaps some are useful. ;-) > > > > > > > > > > > > o Increase the memory via kvm.sh "--memory 1G" or more. The > > > > > > default is "--memory 500M". > > > > > > > > > > Thanks, this definitely helped. > > > > > > Also, I can go back to 500M if I just keep KFREE_DRAIN_JIFFIES at HZ/50. So I > > > am quite happy about that. I think I can declare that the "let list grow > > > indefinitely" design works quite well even with an insanely heavily loaded > > > case of every CPU in a 16CPU system with 500M memory, indefinitely doing > > > kfree_rcu()in a tight loop with appropriate cond_resched(). And I am like > > > thinking - wow how does this stuff even work at such insane scales :-D > > > > Oh, and I should probably also count whether there are any 'total number of > > grace periods' reduction, due to the batching! > > And, the number of grace periods did dramatically drop (by 5X) with the > batching!! I have modified the rcuperf test to show the number of grace > periods that elapsed during the test. Very good! Batching for the win! ;-) Thanx, Paul